[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.