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