Check only our VG name when testing if VG exists

Rather than calling "vgs" we should just call
"vgs <cinder-vg-name>" which returns only information about the VG
we are querying.

Also hardens test command string matching to error on unexpected
calls.

Change-Id: I25580e34b6589e161a51274f22dbb9ed922e43ed
Merge "Check only our VG name when testing if VG exists"