healthchecks: Handle no such host from Network Check and show MetaApiConnErr as WARNING.

[franciscovalentecastro]

When verifying connection to endpoints during Network Check execution (see network_check.go), if the endpoint doesn't exist it shows the following error (see #1630) :

[Network Check] Result: ERROR, Detail: Get "http://metadata.google.internal/": dial tcp *:

This PR does the following changes :

• Handle no such host from Network Check and present FAILURE (log and monitoring api endpoints) or WARNING (not required endpoints for Ops Agent execution) when appropiate.
• Show MetaApiConnErr as WARNING (not fatal error).

Description

Related issue

How has this been tested?

Checklist:

• Unit tests
  • Unit tests do not apply.
  • Unit tests have been added/modified and passed for this PR.
• Integration tests
  • Integration tests do not apply.
  • Integration tests have been added/modified and passed for this PR.
• Documentation
  • This PR introduces no user visible changes.
  • This PR introduces user visible changes and the corresponding documentation change has been made.
• Minor version bump
  • This PR introduces no new features.
  • This PR introduces new features, and there is a separate PR to bump the minor version since the last release already.
  • This PR bumps the version.