Update multiple backend configuration in tempest script

Currently when we configure the tempest with multiple LVM
backends the backend names are created as LVM_iSCSI and
LVM_iSCSI_2 which fails since the backends created by cinder
are lvmdriver-1 and lvmdriver-2.

This patch updates the backend names. Added support for
CINDER_ENABLED_BACKENDS and added todo to remove
CINDER_MULTI_LVM_BACKEND once its formally removed

Closes-Bug:#1369946
Closes-Bug:#1369942

Change-Id: If44f5eb206616afb0fbaf333f3fa6a296d4650cd
Merge "Update multiple backend configuration in tempest script"