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

(cherry picked from commit 94940882f542c701ab6de27ddd0e51857707fc2b)

Conflicts:
	manifests/volume/rbd.pp

Change-Id: I036ea1bf9dd0c6941d7f02038b39a7f293d68ffe
Merge "volume/rbd: Re-enable conditional for string parameters" into stable/havana