Add a custom resource to set up PostgreSQL 12

Supports both primary and replica. The access rules and firewall have to
be set up outside of the custom resource, so they are part of the
recipes instead

Refs #160

site-cookbooks/kosmos-postgresql/attributes/default.rb

@@ -0,0 +1,3 @@
+# This is set to false by default, and set to true in the server resource
+# for replicas.
+node.default['kosmos-postgresql']['ready_to_set_up_replica'] = false