Added JoyentNodeDriver.ex_get_node()

Joyent didn't have a method for get only one Node.

Because I doesn't want to make a for loop for find a node, I added this method.
Error raising is better too with that, a real Joyent's error message is now raised.

Closes #421

Signed-off-by: Tomaz Muraus <tomaz@apache.org>