Don't touch/set CINDER_ENABLED_BACKENDS when CONFIGURE_GLUSTERFS_CINDER=False

[Deepak C Shetty]

As discussed, current code defaults setting CINDER_ENABLED_BACKENDS to lvm and glusterfs, which is OK when CONFIGURE_GLUSTERFS_CINDER=True, but if its set to False, don't modify CINDER_ENABLED_BACKENDS, just leave it to the devstack's default.

[Deepak C Shetty (dpkshetty)]

I set priority to High, since once this is fixed, only then it makes sense to document all the different ways of using devstack-plugin-glusterfs in the README.md of the project's git source.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/228405
Committed: https://git.openstack.org/cgit/openstack/devstack-plugin-glusterfs/commit/?id=f53859f89e78cce80c0e9709a1ae769fec76e252
Submitter: Jenkins
Branch: master

commit f53859f89e78cce80c0e9709a1ae769fec76e252
Author: Bharat Kumar Kobagana <email address hidden>
Date: Mon Oct 19 00:44:17 2015 +0530

    Refactoring devstack-plugin-glusterfs code

    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