Don't always override CINDER_* env variables
Don't always override CINDER_* env variables

We just want to set new defaults, not force the
variables. This way we can mix ceph and other
backends with like:

CINDER_ENABLED_BACKENDS=lvm:lvm-driver,ceph:ceph-driver

Change-Id: I99cc047bce2f584c2b54196a9c84267d2c686231

openstack/devstack-plugin-ceph,ajarr/devstack-plugin-ceph