Initial GENEVE TUNNEL implementation and tests.

Initial GENEVE TUNNEL implementation and tests.

Notes on this first implementation:
* First version of the implementation does NOT support GENEVE OPTIONS
HEADER: it isn't well understood what the purpose of the OPTIONS will be and/or
what content would be placed in the variable option data;

Once the IETF work will evolve and further information will be available
it could be possible to modify the frame rewrite to contemplate the
actual GENEVE OPTIONS.

Change-Id: Iddfe6f408cc45bb0800f00ce6a3e302e48a4ed52
Signed-off-by: Marco Varlese <marco.varlese@suse.com>