Adds multi-backend support documentation

This patch adds multi-backend support documentation for cinder.

This patch was made with the help of:
https://wiki.openstack.org/wiki/Cinder-multi-backend

Link to the patch:
https://review.openstack.org/#/c/21815

This patch fixes the bug 1155622

Change-Id: If47752782e1b690845d1f393a78f5c9a8e4b9171
Added documentation about GlusterFS volume support.

A small mention of Gluster was added to getstart.xml. A full description
of its options was added to the Block Storage guide. Finally, a table
was modified in the Ops guide to show it now has volume support.

Patchset adds more explanation about sparsed volumes settings.
Merge with master.

Change-Id: I36cb344e63f1bddf4125084e02ac507f56591a11
Fixes: bug #1117767
Note that install guide also works for 12.04

I had feedback to the effect that people were ignoring this
install guide because it appeared not to work for ubuntu 12.04,
which is untrue.

Explictly labelling this means that those looking to run openstack
on Ubuntu's Long Term Supported release will know this guide can
also be used.

Change-Id: I61940132ac2bc26c91dde95713e48123db45fb15
Merge "Added documentation about GlusterFS volume support."
Merge "Note that install guide also works for 12.04"
Updates the incubation page to show the integrated projects.

Change-Id: If434de43e1daa8a3dc2b0f60413394298e902341