add classify session action set-sr-policy-index

add classify session action set-sr-policy-index

This allows to use the classifier to steer source routing packets instead
of using the "sr steer" command.
This way we can steer on anything instead of only the dst ip address.

test:
 * add add_node_next function to the VppPapiProvider class.
 * add simple test scenario using the classifier to steer packets with
   dest ip addr == a7::/8 to the source routing insert node.
 * use new interface indexes (3,4) instead of (0,1) to prevent a cleanup
   conflict with the other tests which attach a specific fib to the
   interface.

The test creates interfaces sepsrated from the other tests to prevent a
conflict in the cleaning of the ip6 fib index 1 which causes vpp not to
be able to find a default route on this table.

Change-Id: Ibacb30fab3ce53f0dfe848ca6a8cdf0d111d8336
Signed-off-by: Gabriel Ganne <gabriel.ganne@enea.com>