BigSwitch: Move config and REST to diff modules

No functionality change. Separates the config,
rest call, and backend server management from
the main plugin.py file. Necessary to make
downstream patches more managable and easier
to review.

Implements: blueprint bigswitch-separate-server-module
Change-Id: Ie1fd18a9d8cde24945513c06f7b62239202258a3
Merge "BigSwitch: Move config and REST to diff modules"