Add QoS backend/rule support table to documentation
Add QoS backend/rule support table to documentation

Added a table in OpenStack Manuals documentation to define
which QoS rules are supported by the Networking back-ends.

Change-Id: Ide62cec53864928cbad6f212e13b8847e660f6fd

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals