rds module: Fix undeclared reference to "params" when setting password in modify mode

While modifying an RDS instance with the 'rds' module, one of the actions was to change the password.

While doing so, this happened:
```
TASK: [Set password replica] ******************************************
failed: [localhost] => {"failed": true, "parsed": false}
invalid output was: Traceback (most recent call last):
  File "/Users/herby/.ansible/tmp/ansible-tmp-1399628204.52-67026896752847/rds", line 1865, in <module>
    main()
  File "/Users/herby/.ansible/tmp/ansible-tmp-1399628204.52-67026896752847/rds", line 393, in main
    params["master_password"] = password
UnboundLocalError: local variable 'params' referenced before assignment
```
The proposed change fixes this issue.