add postgresql role for ens. psql and opt. some dbs and users are set up

Add postgresql role for ensuring postgresql is installed.
Furthermore the role optionally takes some basic configuration to ensure
databases with their owners and users are set up as specified.

This is a requirement for a new netbox role.

roles/postgresql/meta/argument_specs.yaml

@@ -0,0 +1,28 @@
+argument_specs:
+  main:
+    options:
+      postgresql__dbs:
+        type: list
+        elements: dict
+        required: false
+        default: [ ]
+        options:
+          name:
+            type: str
+            required: true
+          owner:
+            type: str
+            required: true
+      postgresql__users:
+        type: list
+        elements: dict
+        required: false
+        default: [ ]
+        options:
+          name:
+            type: str
+            required: true
+          password:
+            type: str
+            required: false
+            default: ""