Cherry-pick #17418 to 7.x: [Metricbeat AWS] check if cpuOptions is nil in DescribeInstances output

[kaiyan-sheng]

Cherry-pick of PR #17418 to 7.x branch. Original message:

I cannot reproduce this error with my AWS account. But based on the error message in the bug report #17392, I believe it is caused by response body of DescribeInstances API has empty cpuOptions:

2020-04-01T12:57:03.827+0200	ERROR	runtime/panic.go:199	recovered from panic while
fetching 'aws/ec2' for host ''. Recovering, but please report this.	{"panic": "runtime error: 
invalid memory address or nil pointer dereference", "stack": 
"github.com/elastic/beats/libbeat/logp.Recover\n\t/go/src/github.com/elastic/beats/libbeat/logp/
global.go:105\nruntime.gopanic\n\t/usr/local/go/src/runtime/panic.go:679\nruntime.panicmem\n
\t/usr/local/go/src/runtime/panic.go:199\nruntime.sigpanic\n\t/usr/local/go/src/runtime/signal_un
ix.go:394\ngh.neting.cc/elastic/beats/x-pack/metricbeat/module/aws/ec2.
(*MetricSet).createCloudWatchEvents\n\t/go/src/github.com/elastic/beats/x-
pack/metricbeat/module/aws/ec2/ec2.go:238\ngh.neting.cc/elastic/beats/x-
pack/metricbeat/module/aws/ec2.(*MetricSet).Fetch\n\t/go/src/github.com/elastic/beats/x-
pack/metricbeat/module/aws/ec2/ec2.go:133\ngh.neting.cc/elastic/beats/metricbeat/mb/module.
(*metricSetWrapper).fetch\n\t/go/src/github.com/elastic/beats/metricbeat/mb/module/wrapper.
go:249\ngh.neting.cc/elastic/beats/metricbeat/mb/module.
(*metricSetWrapper).startPeriodicFetching\n\t/go/src/github.com/elastic/beats/metricbeat/mb/
module/wrapper.go:217\ngh.neting.cc/elastic/beats/metricbeat/mb/module.
(*metricSetWrapper).run\n\t/go/src/github.com/elastic/beats/metricbeat/mb/module/wrapper.go
:201\ngh.neting.cc/elastic/beats/metricbeat/mb/module.
(*Wrapper).Start.func1\n\t/go/src/github.com/elastic/beats/metricbeat/mb/module/wrapper.go:1
40"}

This PR is to fix the error by checking if cpuOptions is nil first before evaluating cpuOptions. Similarly, I also added the check for Placement field.

[elasticmachine]

Pinging @elastic/integrations-platforms (Team:Platforms)