datapath: Move tun_key out of struct phy and union.

Tunneling metadata is important enough to move out of struct phy
and handled on its own.  This makes it somewhat easier to tell
how well the other structures are packed and also the name shorter.
This also drops the union since it's not needed quite yet.  We
can introduce it back when we have support for IPv6 tunneling.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Kyle Mestery <kmestery@cisco.com>