Extend web-backend configuration to automatically create a Scenario on startup

[iripiri]

related to VILLASframework/web-backend#140

TODOs:

• add extraConfig paramters in values.yaml (see node configuration which is doing something very similar)
• create web-backend-configmap.yaml (see node-configmap.yaml)
• extend web-backend-deployment.yaml to use the configMap (see node-deployment.yaml)