Fix qos rules are not applied on ocata with ovs

If the Open vSwitch agent is being used, set extensions to qos
in the [agent] section of ml2_conf.ini.
see
https://docs.openstack.org/ocata/networking-guide/config-qos.html

Change-Id: I85272b03222775b14a311a110077a8eed736f56c
Closes-Bug: #1676319