Create get_instances_by_name() in rover.

This moves one more EC2 function into the rover section for later
sharing between pave and plow.
Call get_instances_by_name() and handle array back.

Previously the call format could error out if the function returned more
than one instance name.
Merge pull request #23 from MarkBennett/move-ec2-commands-into-rover-functions

Create get_instances_by_name() in rover.