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.

To ensure proper isolation within a single bridge, NORMAL action
can't be used any more as it floods unknown unicasts on all
bridges ports. It is replaced by a learn action that dynamically
sets-up flows when packets are recieved from tunnel ports. As mac
address are learnt as explicit flows (in table 20), we can use a
default action to flood unknown unicasts to the right set of ports,
as broadcasts and multicasts packets. (see http://bit.ly/14grD6y
for a more detailled explanation of the flow logic)

Another alternative could have been to use distinct bridges for
each tunnel type (whithout modifying the current flow logic).
But previous alternative may be preferable as it paves the way for
new tunneling optimisations (like RPC based mac learning and
partial-mesh flooding proposed in bp/l2-population)

Change-Id: I1dfe74f96680c2c6fe4d8d4aac4821c6b020c005
Closes-Bug: #1196963