First functional version of Compute component with IETF L2VPN REST-API endpoint for OSM.

Common:
- added chk_attribute to type_checkers module.
- recovered run_tests_locally.sh

Compute:
- updated requirements.in
- updated Config.py
- updated RestServer
- implemented IETF-L2VPN NBI plugin
- implemented unitary tests for IETF-L2VPN NBI plugin including a Mock OSM client, and Mock Context and Service components