[VPP-231] There should be SNAT commands in CLI

[vvalderrv]

• Is NAT supported on VPP? There should be a command for SNAT in CLI

SNAT C-API needs to be added to enable configuration of SNAT using Honeycomb/GBP.
There should be support in VPP for SNAT configuration.
commands:
set interface snat set interface snat in <intfc> out <intfc> [del]
snat add address snat add addresses <ip4-range-start> [- <ip4-range-end>]
show snat show snat

Is everything done for SNAT in VPP? What is the current developer state of SNAT? Has this function been tested and documented (e.g. some user guide, examples)?