Allow Pushy Server to optionally use a different username for establishing database connections

[zanecodes]

Description

This change allows Pushy Server to optionally use a different username for establishing database connections than is used within database queries. This is necessary when using Azure Database for PostgreSQL as an external database.

Issues Resolved

https://github.com/chef/chef-server/issues/1559

Check List

• [ ] New functionality includes tests
• [ ] All tests pass
• [x] All commits have been signed-off for the Developer Certificate of Origin. See https://github.com/chef/chef/blob/master/CONTRIBUTING.md#developer-certification-of-origin-dco