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

Context service:
- corrected port numbers

Device service:
- corrected port numbers
- upgraded gitlab-ci configuration

General changes:
- implemented data type checker class
- improved database Entity/Collection/Attributes API
- implemented grpc message parsing assertions
- integrated code coverage analyzer and reporter with code testers
- added example topology NSFNET

Common to device & context services:
- improved database context API and its tests

Context service:
- updated context proto file: added AddLink/DeleteLink RPCs, added LinkId to Link
- minor changes

Device service:
- implemented device service functionality
- improved device client
- updated genproto script
- updated device proto file: configure request becomes a Device message
- updated context proto file: operational status: added KEEP_STATUS, changed DISABLED
- moved device tests to global scripts
- extended unitary tests
- temporarily disabled integration tests

- definition of context API
- implementation of inmemory and Redis backend engines
- implementation of unit tests for both engines

Preliminary unfinished version of device service and Context API based on Redis. To be finished, it is just a backup.