This patch introduces a new configuration parameter (knob)
CONFIGURE_GLUSTERFS_CINDER to enable/disable glusterfs as a
backend for Cinder. And added below error conditions:
* glusterfs plugin enabled but CONFIGURE_GLUSTERFS_CINDER set to
False
* CONFIGURE_GLUSTERFS_CINDER set to True, but CINDER_ENABLED_BACKENDS
doesn't have at least one backend of type glusterfs.
Change-Id: Ic802a2f2bbbdc1af04fd1220d2055be1e853fef8