Add VXLAN backend

VXLAN datapath is supported by the kernel thereby reducing the
overhead of TUN device and userspace switching. This patch takes
advantange of DOVE extensions and does not use multicast for port
flooding.

Fixes #18
You can run multiple flannel instances (processes). In that case, each will be pointing to a different etcd-prefix (that contains the config). If multiple instances are all using VXLAN backend, they would need to specify a unique VXLAN ID (VNI) for each one. That VNI is what's after that dot in the interface name (e.g. VNI=42 -> flannel.42).

Since we can end up deleting interfaces, there is a potential for deleting the a "live" (wrong) interface in case of a mis-configuration.