Add VXLAN tunneling support for the ML2 plugin

Add an ML2 Type Driver for VXLAN networks to allow the creation
of VXLAN networks with the OVS agent.

Change-Id: I7fcc384cc44d5adc752510847d8ba5f46bbb79fb
Implements: blueprint ml2-vxlan
Add gre tunneling support for the ML2 plugin

This patch add the type_driver GRE to enable the creation of GRE
tunnels with the OVS agent.
No Endpoints ID are managed. Only Endpoint IP are stored in DB,
and this IP is proposed as an endpoint ID for the OVS agent.
It also fixes the bug 1201471.

Implements: blueprint ml2-gre
Change-Id: I1a33a4bd3ebc4c97eecf17a59ce16b8c2066ec66
Merge "Add gre tunneling support for the ML2 plugin"
Merge "Add VXLAN tunneling support for the ML2 plugin"