Don't zero out thin provisioned LV's on delete

Thin provisioned LV's don't need secure delete to protect
from data leakage.  Also, zeroing these out kinda defeats
the purpose of using thing provisioning.

This patch add a check for the lvm type and if it's thin simply
returns from the lvm.clear_volume() method.

Conflicts:
	cinder/tests/test_volume.py
	cinder/volume/drivers/lvm.py

Change-Id: Ie6764209018152565295291efc6fbba553698ae6
Closes-Bug: #1240299
(cherry picked from commit de4392314f9b0def8fab65679ec5668aec98fbee)