Bugs and Enhancements to SB REST and SFF/SF Data Plane

- Expanded and cleared considerably the Python refernce implementation.
  - There are now mseparate modules for NSH encode, decode
  - Module for SFF client
  - Self contained module for SFF
  - Module of SFF with REST
  - An agent based module that creates SFF on the fly, meaning no
    pre-configuration needed. This will probably become the future
    of data plane side ODL SFC.
  - In next commits will enahnce agent to create SFs on the fly as well.
  - SFF now uses a dictionary indexed by sfp-id and sfp-index to
    lookup next SF/SFF. This is an considerable improvement over the
    preivous method.

 - Fixed an issue on the SB REST where SFP were not communicated down
 - Fixed an issue where SFPs were deleted more than once. Not really
   a biggie given DELETE is idempotent but still worth it.
 - Service locator Yang now can be configured with a local function

Change-Id: Ic89f175781f29eea7b2aeef7e9e748cc624be6bb
Signed-off-by: Reinaldo Penno <rapenno@gmail.com>