- Added subfolder rest_server/nbi_plugins/ietf_network_slice
- Modified and created classes to register new URLs in the REST server
- Skeleton for methods GET/POST/DELETE
- Refactoring: moved "tools" subfolder one level up

- Migrated to use new generic gRPC servicer
- Migrated to use new generic Rest servicer
- Migrated to use new settings framework
- Minor code formatting/styling

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

- Added REST API Server to Compute component
- Factorized get settings from environment variabes into common method get_setting

This reverts merge request !3

Move database to common folder, since it will be used also by device service, and implement very minor changes.