rework of the nova inventory

 - you can refer to instances by their nova name in ansible{-playbook} calls
 - you can refer to single tenants, regions and openstack environments in ansible{-playbook} calls
 - you can refer to a hostgroup when you pass the arbitrary --meta group= in "nova boot"
 - it caches the state of the cloud => consequent calls are fast
 - it tries to guess ansible_ssh_user based on name of image ('\cubuntu' -> 'ubuntu', '\ccentos' -> 'cloud-user', ...)
 - allows to access machines by their private ip [1]
 - it will work with no additional configuration, just handling single tenant from set OS_* environment variables (just like python-novaclient).
 - you can choose to heavy-configure it for multiple environments
 - configured from simple YAML (I dislike ConfigParser). See nova.yml in the diff

[1] I took few ideas and some code from other pull requests
 - https://github.com/ansible/ansible/pull/8657 by Monty Taylor
 - https://github.com/ansible/ansible/pull/7444 by Carson Gee