addons: bridge: add new policy vxlan_bridge_igmp_snooping_enable_port_mcrouter

addons: bridge: add new policy vxlan_bridge_igmp_snooping_enable_port_mcrouter

if igmp snooping is enabled on a vxlan bridge and if the
vxlan_bridge_igmp_snooping_enable_port_mcrouter is turned on, ifupdown2
will automatically enable multicast router on the vxlan brport unless
this attribute was provided by the user. The policy is enabled by default.

The policy can be disabled as follow:
{
    "bridge": {
	"module_globals": {
	    "vxlan_bridge_igmp_snooping_enable_port_mcrouter": "no"
	}
    }
}

auto br0
iface br0
      bridge-ports vx42
      bridge-mcsnoop yes

auto vx42
iface vx42
      vxlan-id 42

$ ifreload -ad
will show that the config is applied

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