Use encrypted value for gitlab_rails['db_password']

Summary

Currently, gitlab_rails['db_password'] is plaintext in both gitlab.rb and database.yml. It would be more secure if we could get this password encrypted.

Proposal

Use the encrypted password for gitlab_rails['db_password'] in gitlab.rb and database.yml.

References

Customer ticket: https://gitlab.zendesk.com/agent/tickets/127136 (internal use)

Related to #3290 (closed)