Current check uses cinder.utils.has_ceph_backend template which
returns bool-like string values("true" or "false"), and consider it
as bool type. So it is always true regardless of whether there is
ceph backend or not.
To fix this, this change uses string comparision.
Change-Id: Ie2e54c00d536874562eb93e70a2836cac102c992
93bad61c4e