Begin is_service_enabled() cleanup

This converts the special cases in the is_service_enabled() function to call
individual functions declared by the projects.  This allows projects that
are not in the DevStack repo and called via the extras.d plugin to handle
an equivalent service alias.

* Ceilometer
* Cinder
* Glance
* Neutron
* Nova
* Swift

TODO: remove the tests from is_service_enabled() after a transition period

Patch Set 2: Rebased

Change-Id: Ic78be433f93a9dd5f46be548bdbd4c984e0da6e7
remove build_tempest

we haven't actually used this script in about a year, in favor of
the actual in tree lib/tempest.

Change-Id: I9d78b395846ebe833a38ba50edae226040cd7f45
Merge "remove build_tempest"
Merge "Begin is_service_enabled() cleanup"
Merge branch 'master' of https://github.com/openstack-dev/devstack