Add MVRP support to LinuxBridge plugin.

Enabled in /etc/neutron/plugins/linuxbridge/linuxbridge_conf.ini:

    [agent]
    mvrp = True

LinuxBridge will issue commands to configure network interfaces with
MVRP enabled, like so:

    ip link add link eth1 name eth1.100 type vlan id 100 mvrp on

Requies linux >= 3.9.8, and iproute2.  See contrib/MVRP/README.rst for details.

Implements: blueprint mvrp-linuxbridge
Change-Id: Ib0efeafca53dc801187c0655aa03244a6e5949f7
merged from bp/mvrp-linuxbridge

Change-Id: I15529c8b384797e66da7151d6ffde7faf4328393
merged from bp/mvrp-linuxbridge

Change-Id: Ieb0561ce2319d22204ab45eab54529b634d47839