This patch does the following:
* Moved variable setting part from plugin.sh to settings file.
* By default all Gluster-[Glance|Nova|Cinder|Manila] integrations
disabled. Look at README file to know how to enable.
* Keeping CINDER_ENABLED_BACKENDS intact when
CONFIGURE_GLUSTERFS_CINDER=False (Bug #1500381)
* Renamed CONFIGURE_GLUSTERFS_BACKUP to
CONFIGURE_GLUSTERFS_CINDER_BACKUP
* Avoid setting unnecessary variable when speicific configuraiton
is disabled.
* Documented the steps to enable in README.md file of this project.
Closes-Bug: #1500381
Depends-On: Id6967499729dfaa12d8a338d8f72471ebc90fb0d
Change-Id: I36f35f8af8e3491c8b0cffb40ba3980b7beab5df
f53859f89e