Brick LVM: Optimize get_volume

When we only want the LV information for a single volume, call
"lvs" with the volume name specified.

This should reduce unnecessary load of calling "lvs" for the
whole VG.

Related-Bug: #1373513

Change-Id: Ifbb679e9160f44fe52ecba7bf19aa0eb5bb133f1
Merge "Brick LVM: Optimize get_volume"