Add cinder multi backend support to devstack

* add CINDER_MULTI_LVM_BACKEND parameter
* if CINDER_MULTI_LVM_BACKEND = True, creates 2 LVM volumes
    - Name : stack-volumes and stack-volumes2
    - Size : 5GB for each of them

Change-Id: I6e03b96e6b2aed0f0c166472300139fba9eaa68b
Merge "Add cinder multi backend support to devstack"