Add fields labels

[urso]

The add_labels processor was more similar to the fields setting.
This change introduces the add_fields processor to behave like the
fields setting, but changes/limits the add_labels procesor to the use
case of adding a flat set of labels to ECS compatible events.

The add_labels processor will:

• flatten keys by joining nested names with dots
• always writes labels to the events 'labels' field. The target field is
  not configurable and labels can not be written to the root of the
  document

[urso]

jenkins, test this

[ph]

LGTM, will need rebase.

The error is related to TLS on hearthbeat, first time I've seen it.

23:20:25 --- FAIL: TestHTTPSx509Auth (1.32s)
23:20:25     <autogenerated>:1: 
23:20:25         	Error Trace:	mapvaltest.go:40
23:20:25         	Error:      	mapval could not validate map
23:20:25         	Test:       	TestHTTPSx509Auth
23:20:25         	Messages:   	13 errors validating source: 
23:20:25         	            	(common.MapStr) (len=4) {"error":{"message":"Get https://127.0.0.1:45821: read tcp 127.0.0.1:39022-\u003e127.0.0.1:45821: i/o timeout (Client.Timeout exceeded while awaiting headers)","type":"io"},"monitor":{"duration":{"us":1013787},"ip":"127.0.0.1","status":"down"},"tcp":{"rtt":{"connect":{"us":2023}}},"url":{"domain":"127.0.0.1","full":"https://127.0.0.1:45821","port":255,"scheme":"https"}}
23:20:25     <autogenerated>:1: 
23:20:25         	Error Trace:	mapvaltest.go:48
23:20:25         	Error:      	Received unexpected error:
23:20:25         	            	@path 'error': unexpected field encountered during strict validation
23:20:25         	Test:       	TestHTTPSx509Auth
23:20:25     <autogenerated>:1: 
23:20:25         	Error Trace:	mapvaltest.go:48
23:20:25         	Error:      	Received unexpected error:
23:20:25         	            	@path 'monitor.status': objects not equal: actual(down) != expected(up)
23:20:25         	Test:       	TestHTTPSx509Auth
23:20:25     <autogenerated>:1: 
23:20:25         	Error Trace:	mapvaltest.go:48
23:20:25         	Error:      	Received unexpected error:
23:20:25         	            	@path 'tls.rtt.handshake.us': expected this key to be present
23:20:25         	Test:       	TestHTTPSx509Auth
23:20:25     <autogenerated>:1: 
23:20:25         	Error Trace:	mapvaltest.go:48
23:20:25         	Error:      	Received unexpected error:
23:20:25         	            	@path 'http.rtt.validate.us': expected this key to be present
23:20:25         	Test:       	TestHTTPSx509Auth
23:20:25     <autogenerated>:1: 
23:20:25         	Error Trace:	mapvaltest.go:48
23:20:25         	Error:      	Received unexpected error:
23:20:25         	            	@path 'http.rtt.content.us': expected this key to be present
23:20:25         	Test:       	TestHTTPSx509Auth
23:20:25     <autogenerated>:1: 
23:20:25         	Error Trace:	mapvaltest.go:48
23:20:25         	Error:      	Received unexpected error:
23:20:25         	            	@path 'http.rtt.write_request.us': expected this key to be present
23:20:25         	Test:       	TestHTTPSx509Auth
23:20:25     <autogenerated>:1: 
23:20:25         	Error Trace:	mapvaltest.go:48
23:20:25         	Error:      	Received unexpected error:
23:20:26         	            	@path 'tls.certificate_not_valid_before': expected this key to be present
23:20:26         	Test:       	TestHTTPSx509Auth
23:20:26     <autogenerated>:1: 
23:20:26         	Error Trace:	mapvaltest.go:48
23:20:26         	Error:      	Received unexpected error:
23:20:26         	            	@path 'tls.certificate_not_valid_after': expected this key to be present
23:20:26         	Test:       	TestHTTPSx509Auth
23:20:26     <autogenerated>:1: 
23:20:26         	Error Trace:	mapvaltest.go:48
23:20:26         	Error:      	Received unexpected error:
23:20:26         	            	@path 'http.response.status_code': expected this key to be present
23:20:26         	Test:       	TestHTTPSx509Auth
23:20:26     <autogenerated>:1: 
23:20:26         	Error Trace:	mapvaltest.go:48
23:20:26         	Error:      	Received unexpected error:
23:20:26         	            	@path 'http.rtt.response_header.us': expected this key to be present
23:20:26         	Test:       	TestHTTPSx509Auth
23:20:26     <autogenerated>:1: 
23:20:26         	Error Trace:	mapvaltest.go:48
23:20:26         	Error:      	Received unexpected error:
23:20:26         	            	@path 'error.message': unexpected field encountered during strict validation
23:20:26         	Test:       	TestHTTPSx509Auth
23:20:26     <autogenerated>:1: 
23:20:26         	Error Trace:	mapvaltest.go:48
23:20:26         	Error:      	Received unexpected error:
23:20:26         	            	@path 'http.rtt.total.us': expected this key to be present
23:20:26         	Test:       	TestHTTPSx509Auth
23:20:26     <autogenerated>:1: 
23:20:26         	Error Trace:	mapvaltest.go:48
23:20:26         	Error:      	Received unexpected error:
23:20:26         	            	@path 'error.type': unexpected field encountered during strict validation
23:20:26         	Test:       	TestHTTPSx509Auth
23:20:26 FAIL