Add a postgresql_client role

The role is empty but is used to explicitly define servers that have
access rights to all PostgreSQL databases and users

roles/postgresql_client.rb

@@ -0,0 +1,5 @@
+# This role is used by the kosmos-postgresql::default recipe to add access
+# rules to every server that is a PostgreSQL client
+name "postgresql_client"
+
+run_list []