Added NFS max use and oversub ratios

Added the ability to configure additional cinder options for allocating
the over-subscription ratio and for setting the max disk use ratio.

Change-Id: I5387563d5cc869e8be38c8d5a4d9c0d0fc3be469
volume/rbd: Re-enable conditional for string parameters

- According the Michael comments on review¹, the conditional statements
  was re-added (in order to not pass « False » in ini config file).
- Fix spec/defines/* warnings (Space interpreted as grouped expression)

¹https://review.openstack.org/#/c/73892/2/manifests/backend/rbd.pp

Change-Id: I4ef5cbffdc611c011bccb9b400df27830d51343e
Merge "Added NFS max use and oversub ratios"