Make roles respect user specified configuration

The configuration specified by user in gitlab.rb, should have the maximum priority in decision making. omnibus-gitlab should not be overriding that. This issue caused https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30535#note_190274614 (postgres_role enabled repmgr, but we were asking users to disable it in the docs. Even if users disabled it, postgres_role force-enabled it)

Related: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30535 and !3404 (comment 190275198)