Use database attributes for consul watchers

What does this MR do?

Use database attributes for consul watchers

- Refactors the consul watcher recipe for easier reading.
- Fixes the example in the gitlab.rb template which shows configuring a
  complete watcher in the watchers section, when it's really used as
  just a list of strings that indicate which watcher is enabled
- Move the default watcher definition for postgresql into the helper
  because it is directly tied to static values the user cannot change
  such as the name of the failover_pgbouncer template
- Accomodate the potential for folks who may have created their own
  watcher, though if they did it was following a very strict set of
  rules as the ERB template path was hardcoded.

Closes https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/6562

Closes https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/3792

Closes https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/6587

Changelog: changed

Signed-off-by: Robert Marshall <rmarshall@gitlab.com>

Related issues

Closes #6562 (closed)

Closes #3792 (closed)

Closes #6587 (closed)

Checklist

See Definition of done.

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