That won't work. You NEED to use one of the login modules supplied in the connector package or write your own that provides a PasswordCredential. The PasswordCredential includes info about which ManagedConnectionFactory it is for, without it the mcf has nothing to go on.
If you want to use the user/pw from the UserRolesLoginModule you can set up another domain for the db with the CallerIdentityLoginModule. I think it will construct a PasswordCredential using the user/pw from the existing security context, presumably supplied by the UserRolesLoginModule.