Use roles for PG HA components

When configuring nodes to perform the roles of consul server, database server, pgbouncer, we should be able to use the roles feature added in EE 10.1 https://docs.gitlab.com/omnibus/roles/

It'll remove boilerplate from the gitlab.rb's of all nodes, and insulate the user from additional services being added in future releases.

Context: https://gitlab.com/gitlab-org/build/team-tasks/issues/14#note_46533034 https://gitlab.com/gitlab-org/build/team-tasks/issues/14#note_46740827 https://gitlab.com/gitlab-org/build/team-tasks/issues/14#note_46805490