grenade: Only 'enable_plugin ironic' if not already in conf
grenade: Only 'enable_plugin ironic' if not already in conf

To support multi-node grenade jobs we need to move enabling of the
ironic devstack plugin out of the ironic grenade settings file
devstack/upgrade/settings. But if we add enabling of the ironic devstack
plugin to project-config it will cause the gate to break as the plugin
will be enabled twice.

This checks to see if the ironic devstack plugin has already been
enabled:
  * If it has already been enabled it will not enable it again.
  * If it has not yet been enabled it will enable it.

This will allow us to update project-config to enable the ironic
devstack plugin and not break the gate.

After all the changes have landed we will propose a follow-up patch to
remove all this additonal logic and the enabling of the ironic devstack
plugin.

Change-Id: I09ed78ef918eb0359045350e89d42c6df9f3e490

NaohiroTamura/ironic,openstack/ironic,SauloAislan/ironic,pshchelo/ironic,pshchelo/ironic,SauloAislan/ironic,NaohiroTamura/ironic,openstack/ironic