addons: bridge: read bridge multicast_v4_queriers from sysfs

addons: bridge: read bridge multicast_v4_queriers from sysfs

➜  ~ ifquery -a
auto lo
iface lo inet loopback

auto eth0
iface eth0 inet dhcp

auto br0
iface br0
      bridge-ports swp2
      bridge-mcqv4src 100=172.16.100.1 101=172.16.101.1

➜  ~ ifup br0 -v
...
info: executing /sbin/brctl showmcqv4src br0
info: executing /sbin/brctl setmcqv4src br0 100 172.16.100.1
info: executing /sbin/brctl setmcqv4src br0 101 172.16.101.1
...
➜  ~ ifquery br0 -c
auto br0
iface br0                                                           [pass]
      bridge-ports swp2                                           [pass]
      bridge-mcqv4src 100=172.16.100.1 101=172.16.101.1           [pass]

➜  ~

Signed-off-by: Julien Fortin <julien@cumulusnetworks.com>