cloud/rds : renaming an rds instance always fails

##### Issue Type:

Bugfix Pull Request

##### Ansible Version:

ansible 1.6 (rds_modify 632eb18) last updated 2014/04/18 17:39:06 (GMT +000)

##### Environment:

N/A

##### Summary:

Using the RDS module with command=modify and apply_immediately=yes to rename an instance (setting new_instance_name) will currently always fail. When this module was modified recently it wasn't tested with renaming an instance. A call to get_current_resource fails simply because it's trying to look up details of the old instance name, not the new name that the instance was just renamed to.

##### Steps To Reproduce:

An rds task with the following properites set:

    command=modify
    apply_immediately=yes
    new_instance_name=some_new_instance_name

##### Expected Results:

No error should be thrown.  The instance is actually renamed but Ansible throws an error.

##### Actual Results:

    failed: [test_host] => {"failed": true}
    msg: DBInstance starting_test_instance not found.