JCLOUDS-703 removing imageSpaceGb from machineType

See ticket 703. MachineType requests no longer have a value for
imageSpaceGb. This has been reported as a bug.

https://code.google.com/p/google-compute-engine/issues/detail?id=111

If this is no longer to be reported, seems sensible to remove the concept
and simplify MachineType.