Support Ironic in devstack gate

Now Devstack can deploy Ironic baremetal provisioning
service, so it's possible to test it using gate tests.

This patch enables Ironic in devstack gates if the
DEVSTACK_GATE_IRONIC variable is set to 1.

Change-Id: I56ea3a3e2576f5488a43ca4ece35a5a250cef04c
Merge "Support Ironic in devstack gate"