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.

Change-Id: Id6ff85101f85e70575ba244c2df7aca0196cf224
(cherry picked from commit d030c5b10e9e8d73d967562259e7db6146347108)
Merge commit '940f363dafd8a511e0c37b8a1ce1370e36c5a835' into x-release-folsom