Improve LVM Cinder Volume Creation

- The snapshot_autoextend_* configuration should be done only if the
  /etc/lvm/lvm.conf file is present on the server.
- We need to repeat this step after the installation of the lvm2 package.

Change-Id: I4a7e5b9d48c338e19bc86edec967e41029a8a0ec
Signed-off-by: Gael Chamoulaud <gchamoul@redhat.com>
Merge "Improve LVM Cinder Volume Creation" into icehouse