templates: use hostPath for DB deployment

* Makes DB persistence path configurable. A reana-cluster.yaml file
  should be created and configured correctly.

* Prevents DB to be destroyed when running locally or deploying on
  a single node cluster. This is NOT save when deploying on a multi
  node deplyoment since DB pod could be redeployed for different
  reasons to other VMs, loosing all database content. This is ok
  since multi node setups will be in production and they will use
  DBoD (closes reanahub/reana-workflow-controller#52).

reana_cluster/backends/kubernetes/k8s.py

@@ -179,6 +179,10 @@ def generate_configuration(cls, cluster_spec, production=False):
                     backend_conf_parameters['CEPHFS_MONITORS'] = \
                         cluster_spec['cluster'].get('cephfs_monitors')
 
+                if cluster_spec['cluster'].get('db_persistence_path'):
+                    backend_conf_parameters['DB_PERSISTENCE_PATH'] = \
+                        cluster_spec['cluster'].get('db_persistence_path')
+
                 # Would it be better to combine templates or populated
                 # templates in Python code for improved extensibility?
                 # Just drop a .yaml template and add necessary to config.yaml

reana_cluster/backends/kubernetes/templates/deployments/db-template.yaml

@@ -26,7 +26,8 @@ spec:
           value: reana
         volumeMounts:
           - name: data
-            mountPath: /var/lib/pgsql/data
+            mountPath: /var/lib/postgresql/data
       volumes:
         - name: data
-          emptyDir: {}
+          hostPath:
+            path: {{DB_PERSISTENCE_PATH}}

reana_cluster/configurations/reana-cluster-dev.yaml

@@ -10,6 +10,7 @@ cluster:
   version: "v1.9.4"
   db_config: &db_base_config
     - REANA_SQLALCHEMY_DATABASE_URI: "postgresql+psycopg2://reana:reana@db:5432/reana"
+  db_persistence_path: "/reanadb"
 
 components:
   reana-workflow-controller:

reana_cluster/configurations/reana-cluster.yaml

@@ -3,7 +3,7 @@ cluster:
   version: "v1.9.4"
   db_config: &db_base_config
     - REANA_SQLALCHEMY_DATABASE_URI: "postgresql+psycopg2://reana:reana@db:5432/reana"
-  root_path: "/tmp/reana"
+  root_path: "/reana"
 
 components:
   reana-workflow-controller: