Report FQDN on host.name enabled by a feature flag

[AndersonQ]

Describe the enhancement:

Following up on elastic/beats#1070, and elastic/ecs#2122 the Elastic Agent should be able to report the FQDN on host.name. This should be configurable by a feature flag on the agent policy.

Describe a specific use case for the enhancement or feature:
There are cases in which different hosts share the same hostname but have different domains. Currently they have the same host.name values, which isn't ideal. Allowing to populate host.name with the FQDN solves that problem. ALso having it behind a feature flag won't make it a breaking change as the user will have the option to enable it or not.

Implementation details

Feature flag generic config:

agent:
  features:
    feature_1:
      enabled:
      config1:
      config2:
      ...
      configN:
    feature_2:
      enabled:
      config1:
      config2:
      ...
      configN:
    ...
    feature_N:
      enabled:
      config1:
      config2:
      ...
      configN:

Feature flag for FQDN:

agent.features.fqdn.enabled: true|false

Related Issues:

• Blocked by: Report FQDN go-sysinfo#144

[softengchick]

@AndersonQ - Endpoint will need to be aware of this feature flag as well. How do you plan on adding it to the configuration. @brian-mckinney - since he is the expert in this area for Endpoint. I see your other PR now!! THANKS!!!

[cmacknz]

The control protocol PR adding the feature flags is elastic/elastic-agent-client#56 I will add Brian as a reviewer.