This introduces a new configuration parameter neutron_enable_qos to
be able to enable the Neutron QoS service plugin.
More details about the Neutron QoS service plugin are available at:
http://docs.openstack.org/liberty/networking-guide/adv-config-qos.html
Change-Id: I8525bf4dce5f1e225f72a4e1c3760b64a36b17f6
Closes-bug: #1593183
Implements: bp netowrking-qos