[google|compute] Add disk_size_gb to 'Image' model

Added new field to Image model, named diskSizeGb, which shows the
size of the image when it is restored to a persistent disk, in GB.
Add support for tags for Data Pipeline
Merge pull request #2965 from frodenas/gce_images

[google|compute] Add disk_size_gb to 'Image' model
Merge pull request #2973 from accardi/support_tags_for_datapipeline

Add support for tags for Data Pipeline