[google compute] improve MachineType (size) coverage of GCE API

Minor improvements to GCE Machine Types (sizes in libcloud parlance) to match GCE API.
 * added missing resource attributes to `GCENodeSize`
 * hiding some parameters from `extra` since they've been removed (namely `imageSpaceGb`)
 * minor change to the disk.size value based on presence/absence of attributes

Updated test fixture and test.
@runseb - no, you can't change parameters on a GCE MachineType. These are analogous to AWS "sizes" like x2.large, c2.medium (or whatever they are).  They are static descriptions of the "size" of VM.

Google has made a change to the API since this code was originally contributed and now the most appropriate way to report a MachineType's disk size is to use `maximumPersistentDisksSizeGb` over the now rarely seen `imageSpaceGb` attribute.