tunnel: Treat in_key=0 the same as a missing in_key.

tunnel: Treat in_key=0 the same as a missing in_key.

The documented behavior of ovs is that a missing key is the
same as a zero key.  However, the tunneling code actually treated
them differently.  This could cause problems with tunneling modes
such as vxlan which always have a key.  Specifically, a tunnel with
no key configured, would send have to send traffic with a key of
zero.  However, the same tunnel would drop incoming traffic with a
zero key because it was expecting there to be none at all.

Signed-off-by: Ethan Jackson <ethan@nicira.com>