NSH unit test cases using encap and decap actions

With the support of generic encap and decap actions for Ethernet and NSH
it is now possible to build test cases that mimic realistic OVS
configurations and OF pipelines for Service Function Chaining. Packets
are being encapsulated in NSH, forwarded based on NSH headers, sent over
Ethernet links and VXLAN-GPE tunnels, and decapsulated at the end of
a service chain.

Signed-off-by: Jan Scheurich <jan.scheurich@ericsson.com>
Signed-off-by: Yi Yang <yi.y.yang@intel.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>