added check to make sure that the volume group variable exists since the variable is needed for cinder lvm but not netapp

(cherry picked from commit 55d3cc09f3f4470b2e6ea82a6286b1336f83a9e6)
Merge pull request #231 from claco/issue-165-backport

added check to make sure that the volume group variable exists since the variable is needed for cinder lvm but not netapp