[mount] Add documentation for fstab option
[mount] make sure fstab exists

This is mainly to prevent from failing with no such file or directory
during module execution.
Merge pull request #8223 from renard/create-fstab-if-not-exists

Create fstab if not exists
Changed auth args in openstack modules
Nova compute fixes
Added region to nova keypair, fixed example
Fixes to glance image
Added nova security groups
Added cinder volume
Added nova snapshot
Added nova volume
Added swift container
Added swift object
Added nova flavors
Added nova quota
Added cinder quotas
Added nova floating ips
Moving keystone to three modules
Fixed quantum subnet
Fixes to quantum router