Add check for snapshot to Brick LVM.

Adds a check to the LVM code in brick to see if
a snapshot exists.  This implements the incorrectly
named "is_busy" method that exists in the cinder LVM driver.

Change-Id: Ia0f6cbd3d5b4561c5368545448abaafa67caf05d
Merge "Add check for snapshot to Brick LVM."