config: environment variable name clash breaks config

[diegodelemos]

Stemmed from reanahub/reana#309 (comment)

Problem

A collision between REANA-DB configuration variable for DB port and naming for service discovery through environment variables in Kubernetes.

Example

With an example: If we don't set the REANA_DB_PORT environment variable, Kubernetes will set it as a Docker link (see more here) because (by default) there is a service called reana-db and using the Docker link naming scheme an env variable REANA_DB_PORT is created:

$ env | grep -i reana_db_port=
REANA_DB_PORT=tcp://10.102.63.67:5432

Possible solution

Disable service discovery through environment variables (see kubernetes/kubernetes#60099), remove all our dependencies on it and rely uniquely on DNS service discovery (security advantage, as we don't expose all services in user jobs).

Some occurrences of service discovery through environment variables in REANA:

$ rg SERVICE_HOST
reana-commons/reana_commons/config.py
93:                '{}_WORKFLOW_CONTROLLER_SERVICE_HOST'.format(

reana-workflow-controller/reana_workflow_controller/workflow_run_manager.py
391:                'name': 'REANA_JOB_CONTROLLER_SERVICE_HOST',
$ rg SERVICE_PORT
reana-commons/reana_commons/config.py
97:                '{}_WORKFLOW_CONTROLLER_SERVICE_PORT_HTTP'.format(

reana-workflow-controller/reana_workflow_controller/workflow_run_manager.py
386:                'name': 'REANA_JOB_CONTROLLER_SERVICE_PORT_HTTP',