selftests: forwarding: Allow creation of interfaces without a config file

Some users want to be able to run the tests without a configuration file
which is useful when one needs to test both virtual and physical
interfaces on the same machine.

Move the defines that set the type of interface to create and whether to
create it away from the optional configuration file to the library like
the rest of the defines.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>