Add MPLS recirculation tests

This patch introduces a python script to generate about 1500 tests for
permutations of mpls_push,mpls_pop,dec_mpls_ttl,dec_ttl where
recirculation occurs up to (and including) three times.

Signed-off-by: Joe Stringer <joe@wand.net.nz>
Signed-off-by: Simon Horman <horms@verge.net.au>

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v12
* First post