Set service_available config options in tempest

This commit loops over the list of services that tempest has config
options for in the service_available group and checks if the service
is enabled. Devstack will then set whether or not the service is
configured in tempest.conf.

Change-Id: Ib845d3e098fd3f45c8c26f5696af14cca1534e01
Merge "Set service_available config options in tempest"