[QoS] Add info about rate limiting on router's ports

Qos policy with bandwidth limit rules can be applied on router's
gateway ports to limit only if veth are used to connect it to
namespaces.
This commit updates "ovs_use_veth" config option help message and
qos docs to add such info there.

Change-Id: I69e78bc125d430c933f7576d05308030789cee0a
Related-Bug: #1732852

doc/source/admin/config-qos.rst

@@ -136,6 +136,16 @@ On the network and compute nodes:
       [agent]
       extensions = qos
 
+#. As rate limit doesn't work on Open vSwitch's ``internal`` ports,
+   optionally, as a workaround, to make QoS bandwidth limit work on
+   router's gateway ports, set ``ovs_use_veth`` to ``True`` in ``DEFAULT``
+   section in ``/etc/neutron/l3_agent.ini``
+
+  .. code-block:: ini
+
+      [DEFAULT]
+      ovs_use_veth = True
+
 .. note::
 
    QoS currently works with ml2 only (SR-IOV, Open vSwitch, and linuxbridge

neutron/conf/agent/common.py

@@ -54,10 +54,11 @@ INTERFACE_OPTS = [
                help=_('Name of Open vSwitch bridge to use')),
     cfg.BoolOpt('ovs_use_veth',
                 default=False,
-                help=_('Uses veth for an OVS interface or not. '
-                       'Support kernels with limited namespace support '
-                       '(e.g. RHEL 6.5) so long as ovs_use_veth is set to '
-                       'True.')),
+                help=_("Uses veth for an OVS interface or not. "
+                       "Support kernels with limited namespace support "
+                       "(e.g. RHEL 6.5) and rate limiting on router's gateway "
+                       "port so long as ovs_use_veth is set to "
+                       "True.")),
 ]