replace the neo4j with mongo db

Motivation:

We need to store media data for some of the entities and neo4j is
not design to store images, video, etc.. Hence, to avoid using
multiple dbs, we sort for a solution that can store the metadata
and the media data.

Modification:

- remove neo4j dependency and add mongo
- change the entities details and re-design it to suit the new db
- adjust the api design
- delete all neo4j relationship pojos
- adjust the tests according to the new definitions

Result:

Cerebrum now uses mongo db.

Target: master
4 jobs for master in 1 minute and 25 seconds
Status Name Job ID Coverage
  Build
passed build #283357

00:00:38

 
  Test
failed test #283358

00:00:47

 
  Package
skipped package #283359
docker
 
  Docker
skipped docker_build_push #283360
docker
 
Name Stage Failure
failed
test Test
[ERROR] -> [Help 1]
[ERROR]
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR]
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoFailureException
Cleaning up file based variables
ERROR: Job failed: exit code 1