perf(db): test connection to database fqdn

What does this MR do?

Related to #1006 (closed)

Context

Following some investigation in #1006 (comment 1610928878) we determined that we are able to resolve the address of the Gitlab.com primary database server directly from the registry pods without adding a specific resolver.

Prior to the findings of the investigation, we had decided to use service discovery with an explicit resolver that would resolve the primary database fqdn to an IP we can establish a connection to. This is no longer necessary as shown by the findings. Also initial tests proved we were unable to establish connections with the resolver itself #1006 (comment 1610698801).

What is in this MR?

This MR modifies the existing function that used service discovery to test resolving the primary database hostname. With this MR, the existing function instead now verifies that we are able to connect to and ping the primary database server using its fqdn.

Author checklist

• Feature flags
  • Added feature flag:
  • This feature does not require a feature flag
• I added unit tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines
• For database changes including schema migrations:
  • Manually run up and down migrations in a postgres.ai production database clone and post a screenshot of the result here.
  • If adding new queries, extract a query plan from postgres.ai and post the link here. If changing existing queries, also extract a query plan for the current version for comparison.
  • Do not include code that depends on the schema migrations in the same commit. Split the MR into two or more.
• Ensured this change is safe to deploy to individual stages in the same environment (cny -> prod). State-related changes can be troublesome due to having parts of the fleet processing (possibly related) requests in different ways.

Reviewer checklist

• Ensure the commit and MR tittle are still accurate.
• If the change contains a breaking change, apply the breaking change label.
• If the change is considered high risk, apply the label high-risk-change
• Identify if the change can be rolled back safely. (note: all other reasons for not being able to rollback will be sufficiently captured by major version changes).

If the MR introduces database schema migrations:

• Ensure the commit and MR tittle start with fix:, feat:, or perf: so that the change appears on the Changelog

If the changes cannot be rolled back follow these steps:

• If not, apply the label cannot-rollback.
• Add a section to the MR description that includes the following details:
  • The reasoning behind why a release containing the presented MR can not be rolled back (e.g. schema migrations or changes to the FS structure)
  • Detailed steps to revert/disable a feature introduced by the same change where a migration cannot be rolled back. (note: ideally MRs containing schema migrations should not contain feature changes.)
  • Ensure this MR does not add code that depends on these changes that cannot be rolled back.

Related to #1006 (closed)