Fix overcloud introspection data save

If running 'kayobe overcloud introspection data save' in an environment
where nodes have been registered in the seed's ironic service manually
(rather than through auto-discovery), previously the bifrost ansible
inventory would not have been populated.

This change ensures the bifrost ansible inventory exists prior to
collecting introspection data.

Change-Id: I2ad53aa0db3bab8db6d6d9de4b00c5dac756fb15