support ansible check and diff mode

* Operators would like to see some dry run like features in kolla-ansible.
* Would like to see the return of something like genconfig, where
configs can be generated ahead of time and diffed/reviewed before deploy.
* Also some general discussion in this session on management and scaling
difficulties with kolla.
* Inventory management needs to be more flexible.
* Operations are too slow once you hit about 200 nodes, operators are
finding they have to use manual trickery to divide up their inventories.
* A lot of operations take place when very little has changed config wise.