glusterd/snapshot: verify for lvm commands

On non-Linux platforms we need to verify the
run time availability of LVM specific commands
and fail accordingly with a message.

Change-Id: Ie1e3870648f01ee129e390e2240c66e0c6249b90
BUG: 1061685
Signed-off-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-on: http://review.gluster.org/8165
Reviewed-by: Raghavendra Bhat <raghavendra@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Sachin Pandit <spandit@redhat.com>