Several improvements

See merge request teraflow-h2020/controller!11

Common:
- renamed report_coverage.sh script to report_coverage_all.sh

Common/DatabaseAPI:
- improved root class validations and test units
- added _RootEntity interface
- improved definition of Context database API entities

Context service:
- added AddLink/DeleteLink rpc methods
- temporarily disabled integration tests

Device service:
- applied renamings in service and test units

Common:
- implemented ServiceException class
- updated attributes in entities belonging to Database API
- implemented test method to populate database with example topology

Context service:
- updated genproto script
- implemented report_coverage_context script
- moved specific rest scripts to root folder
- minor arrangements in ContextClient
- updated proto python files
- implemented GetTopology RPC functionality
- implemented unitary testing for GetTopology

Device service:
- updated genproto script

Integrate minor changes in context service into develop

See merge request teraflow-h2020/controller!10

First implementation of device service

See merge request teraflow-h2020/controller!9