Add change_password alias for create_user method

A PUT request to the [RabbitMQ API](http://hg.rabbitmq.com/rabbitmq-management/raw-file/3646dee55e02/priv/www-api/help.html) /api/users/name endpoint works like the [rabbitmqctl](http://www.rabbitmq.com/man/rabbitmqctl.1.man.html) change_passowrd command if the user was previously created.  If the user was not previously created then a new user is added with the specified password.