Fix the warning message of CINDER_MULTI_LVM_BACKEND

CINDER_MULTI_LVM_BACKEND is marked as deprecated and we recommend
to use CINDER_ENABLED_BACKENDS instead now. However current warning
message shows that CINDER_ENABLED_BACKENDS will be removed.
This patch fixes the message.

Change-Id: I3ca25e4273c3abebce1cfefe1ae4af622b71e993
Merge "Fix the warning message of CINDER_MULTI_LVM_BACKEND"