Move ironic to plugin

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

Change-Id: I6e6e0e8b70221e231785ab27e9b5d4836933ac4c
Merge branch 'is-service-enabled' into dt-test
Merge branch 'ironic-plugin' into dt-test
Merge branch 'code-cleanup' into dt-test