[Starcluster] Bug VolumeCreator _request_instance

[Justin Riley]

Added --instance-type option to createvolume command in the github 
code. Will be
included in the next release.

~Justin

Quoting Joachim Van den Bogaert <joachim at ccl.kuleuven.be>:
> With the 64-bit image I use instead, an extra parameter could prevent this
> error
>
>
>
> After I changed the line into
>
>
>
> instance_type='m1.large'
>
>
>
> the volume was created successfully.