Improve provider_location cleanup code for RBD.
Improve provider_location cleanup code for RBD.

The RBD driver does not make use of the 'provider_location' field
but the current cleanup code assumes it does.  Ensure the field
is in use before testing whether or not it needs fixing.

Fixes bug 1083818.

Note: Applying to stable/folsom nova-volume code.  Same fix
proposed to cinder @ https://review.openstack.org/#/c/17023/

Change-Id: I814bd253d2e850534a673985e499f8cdbcebb18e

paulmathews/nova,paulmathews/nova,paulmathews/nova