netifd: bridge: set default value for igmp_snoop

netifd: bridge: set default value for igmp_snoop

When unchecked the igmp snoop option for a bridge by luci, it
just delete the igmp_snooping key from the config file.
So netifd can't change /sys/devices/virtual/net/br-lan/bridge/multicast_snooping from "1" to "0".

Option multicast_querier seems no input entry in luci, but it's
an related option.

This patch will set a default value to false for the bridge
option to fix this bug.

Signed-off-by: Zheng Qian <sotux82@gmail.com>