Initial support for vxlan on linuxbridge.

blueprint vxlan-linuxbridge

Linux kernel starting from 3.7 offer native support for vxlan. This is
initial implementation of this feature in quantum for linuxbridge.

Change-Id: I7c6558c3fd890ca76d5dea02ddab005470aa787c
move tenant_network_type to LINUX_BRIDGE

Change-Id: If5556d43aff888e5714ce373e18686bbe784a3ef