[Agent] required changes for enroll

[ph]

Required Changes:

Without TLS

When you are not using TLS we should display a warning not to use that in production.

With TLS

When enrolling with TLS, we need to receive the CA hash to check the remote certificate. The default command will look like this:

agent enroll -ca_sha256 foobar https://localhost:5601 <enrol_token>

[elasticmachine]

Pinging @elastic/ingest (Project:fleet)