tunneling: Allow matching and setting tunnel 'OAM' flag.

tunneling: Allow matching and setting tunnel 'OAM' flag.

Several encapsulation formats have the concept of an 'OAM' bit
which typically is used with networking tracing tools to
distinguish test packets from real traffic. OVS already internally
has support for this, however, it doesn't do anything with it
and it also isn't exposed for controllers to use. This enables
support through OpenFlow.

There are several other tunnel flags which are consumed internally
by OVS. It's not clear that it makes sense to use them externally
so this does not expose those flags - although it should be easy
to do so if necessary in the future.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>