fix(config): set localhost default value for REANA_HOSTNAME (#630)

This PR is part of harmonizing the treatment of REANA_HOSTNAME
accross all REANA components. You can refer to other PRs below.

reanahub/reana#867
reanahub/reana-server#717

Closes reanahub/reana#865

reana_workflow_controller/config.py

@@ -277,9 +277,12 @@ def _parse_interactive_sessions_environments(env_var):
 REANA_GITLAB_URL = "https://{}".format(REANA_GITLAB_HOST)
 """GitLab API URL"""
 
-REANA_HOSTNAME = os.getenv("REANA_HOSTNAME", "CHANGE_ME")
+REANA_HOSTNAME = os.getenv("REANA_HOSTNAME", "localhost")
 """REANA URL"""
 
+REANA_PORT = os.getenv("REANA_PORT", "30443")
+"""REANA Port"""
+
 REANA_INGRESS_ANNOTATIONS = json.loads(os.getenv("REANA_INGRESS_ANNOTATIONS", "{}"))
 """REANA Ingress annotations defined by the administrator."""