tunnel: Support all combinations of flow-based and specific tunnel matches.

There are 12 possible ways to specify a tunnel (2 * 2 * 3 == 12):

    - Specific in_key or flow-based (2 choices).

    - Specific ip_dst or flow-based (2 choices).

    - Specific ip_src, wildcarded, or flow-based (3 choices).

Until now, only 6 of the 12 possibilities have been supported.  We
have had a couple of requests to add another.  This commit adds all the
possibilities, so that we won't have to add the other 6 one by one.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Requested-by: Thomas Morin <thomas.morin@orange.com>