GCE: Add missing description to GCENetwork

Signed-off-by: Sebastien Goasguen <runseb@gmail.com>

This closes #397
Improve MachineType (size) coverage of GCE API

Signed-off-by: Sebastien Goasguen <runseb@gmail.com>

This closes #396
GCE: improved Images coverage

Signed-off-by: Sebastien Goasguen <runseb@gmail.com>

This closes #395