Fix misleading message when protocol is not set

[dadoonet]

In 7.1.0, when configuring a monitor like this:

heartbeat.monitors:
- type: http
  schedule: '@every 5s'
  urls:
    - 'elasticsearch:9200'

It fails with this error message:

2019-05-27T10:52:35.556Z	ERROR	instance/beat.go:802	Exiting: could not create monitor: job err missing port in address
Exiting: could not create monitor: job err missing port in address

I found it misleading as the port is there obviously. Actually the problem is coming from the protocol which is missing. It should have been:

heartbeat.monitors:
- type: http
  schedule: '@every 5s'
  urls:
    - 'http://elasticsearch:9200'

[elasticmachine]

Pinging @elastic/uptime

[andrewvc]

This was fixed in #13703