Bug 1620879: Combine duplicate "get_allowed_services" method

Two of the server classes had their own implementations of this,
with slightly different logic. Moving it up into the parent class
in order to make sure everything works the same, all the time.

behatnotneeded: Can't test in Behat

Change-Id: I91c205822a18213c475bbef7c784247cebdbd649
(cherry picked from commit ef871685)