Disable IGMP snooping on hybrid Linux bridge

By default, Linux bridge activates the IGMP snooping on his switch.
It is unnecessarily on Linux bridge created by OVS/VIF hybrid vif
drivers because that switch is limited two ports.

That patch also refactoring OVS and IVS plug and unplug unit tests of the
hybrid drivers.

Closes-Bug: #1276038
Change-Id: Ib8bb1874d7a300fddffdc76ba67851109248bebf
Merge "Disable IGMP snooping on hybrid Linux bridge"