Doc needs update for glance datastore backend driver

[Man Li Qi]

According to current code, to enable the glance datastore backend driver function, the following section should be added and configured in glance-api.conf:

[glance_store]
default_store = vsphere
stores = glance.store.vmware_datastore.Store

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/146003

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/146003
Committed: https://git.openstack.org/cgit/openstack/glance/commit/?id=f4674e04a3646770733cacadc6d5defc901d4f10
Submitter: Jenkins
Branch: master

commit f4674e04a3646770733cacadc6d5defc901d4f10
Author: Matt Fischer <email address hidden>
Date: Thu Jan 8 20:18:31 2015 -0700

    Improve documentation for glance_stores

    The documentation for glance_stores was missing any mention of the
    stores option which is required to make a default_store work if it's
    non-default.

    Closes-Bug: #1406182

    Change-Id: I89e8fe2d8b31cd66609fd4132cb263eecb3db4c3