AWS RDS Support - 1

Necessary to add geonode permissions to the postgres user.  In AWS RDS, by default postgres "inherits" from rds_superuser instead of rdsadmin.  Therefore, the postgres user can "inherit" any role, but needs to explicitly "request" it.

Below is what the roles look like after provision.
```
postgres=> \du
                                      List of roles
   Role name   |                   Attributes                   |        Member of        
---------------+------------------------------------------------+-------------------------
 geonode       |                                                | {}
 postgres      | Create role, Create DB                         | {rds_superuser,geonode}
 rds_superuser | Cannot login                                   | {}
 rdsadmin      | Superuser, Create role, Create DB, Replication | {}
 rdsrepladmin  | No inheritance, Cannot login, Replication      | {}

```