Enable GRE and VXLAN with the same ID

Current packet processing in br-tun is based on tun-id,
as a consequence, two networks using different tunnel
types but sharing the same tun-id would not be properly isolated.

This patch changes br-tun packet handling, using several tables
and flow-based mac learning instead of using the normal action.

Another alternative could have been to use distinct bridges for
each tunnel type (whithout modifying the current flow logic).
Firt one was chosen as it opens new perspective to improve
tunnel mangement (like learning mac via RPC or implement
partial-mesh for broadcasts/multicats)

Fixes bug #1196963

Change-Id: If83adafa76afd0f55b8b93f02cebf3d19c0ba683