Fix config group for cinder multi_backend

This commit just updates the config group for the multi_backend option.
Tempest change d5c9602b created a volume-feature-enabled group and moved this
volume there but devstack was never updated with the change.

Closes-Bug: #1276326

Change-Id: Icf2e96783feec4edbd4d477f8492651cd9bb3f01
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 "Fix config group for cinder multi_backend"
Merge "Move ironic to plugin"