openvswitch: Reset tunnel key between input and output.

It doesn't make sense to output a tunnel packet using the same
parameters that it was received with since that will generally
just result in the packet going back to us. As a result, userspace
assumes that the tunnel key is cleared when transitioning through
the switch. In the majority of cases this doesn't matter since a
packet is either going to a tunnel port (in which the key is
overwritten with new values) or to a non-tunnel port (in which
case the key is ignored). However, it's theoreticaly possible that
userspace could rely on the documented behavior, so this corrects
it.

Signed-off-by: Jesse Gross <jesse@nicira.com>