Extract remaining postgres attributes to the correct cookbook

What does this MR do?

This is a refactor of gitlab cookbook to extract remaining pieces of postgresql service in its own cookbook.

This also moves the attributes from node['gitlab']['postgresql'] to a root level.

Part of the effort of #3860 (closed)

Checklist

See Definition of done.

• MR targeting master branch
• MR has a green pipeline on GitLab.com

Reviewer Checklist

In addition to above, reviewer must:

• Pipeline is green on dev.gitlab.org if the change is not touching documentation or internal cookbooks

References

#3860 (closed)