Add support for VXLAN to the Open vSwitch plugin.

Add support for VXLAN to the Open vSwitch plugin.

This commit adds support for utilizing the VXLAN tunneling protocol in versions
of Open vSwitch >= 1.10. This is configurable and will default to GRE if not
configured. As part of this commit, it is possible to configure the UDP port
VXLAN will utilize as well. VXLAN and GRE cannot be configured at the same
time with this patch. 2 new configuration file options are added to the AGENT
section of the config to support this: 'tunnel_type' and 'vxlan_udp_port'.
In addition, the agent no longer makes use of enable_tunneling, as this can
be determined if tunnel_type is set.

Note: The VXLAN functionality utilized here is what is implemented in Open
vSwitch itself, and is different than the VXLAN functionality in the
upstream Linux kernel. The code validates both the userspace and kernel
pieces of OVS to verify if VXLAN functionality can be supported on the
running system

Implements blueprint ovs-vxlan-lisp-tunnel

Change-Id: I45d49d5d6463e574922c7f50d6499c6bdb6c862c