Move ironic to plugin

Also adds an is_ironic_enabled() function to prepare for an upcoming
change in is_service_enabled().

Change-Id: I6e6e0e8b70221e231785ab27e9b5d4836933ac4c
Generate Tempest service list rather than hard-code it

The list of services that Tempest used to set its 'service_available'
config values was hard-coded. To be plugin-friendly have each
service (project) add its name to the TEMPEST_SERVICES variable
and use that for setting the 'service_avilable' values.

Change-Id: I208efd7fd0798b18ac2e6353ee70b773e84a2683
Merge branch 'tempest-service-config' into dt-test
Merge branch 'code-cleanup' into dt-test