Integration of Service service, multiple improvements in Context and Device services, and GitLab CI pipeline integration and functional tests

See merge request teraflow-h2020/controller!12

Common / Context / Device / Service:
- Improvements in unitary testing
- Extended assertion validators tor testing
- Factorized service implementation code to improve testability, reusability and increase code coverage

Factorization of code between services and minor unitary testing improvements. (Not finished, it is just a backup)

Common:
- minor bug resolutions in Context API

Service:
- first functional version of Service (implementation of GetConnectionList postponed)
- minor corrections in service.proto and related files
- implemented unitary tests

Common:
- Improvements in Database API definition and unit tests
- Implemented additional Assertions to be used by unitary tests

Context and Device:
- Improved logging of errors.

Service:
- Implemented service methods (except GetConnectionList). All methods are under active debug.
- Partially implemented unitary tests

Common:
- reorganized Database API to enable extension of Context API with entities (e.g. Service)
- solved minor bugs in Database API

Context and Device services:
- rearranged services and unit tests according to mentioned Context API changes

Service service:
- initial release (under development, this is just a backup of code)

Several improvements

See merge request teraflow-h2020/controller!11