fix instance template interaction with regional disks, make docs more clear about valid values

[danawillow]

fixes #2128

[rileykarson]

These changes LGTM. I'm just curious why we can't use a self_link here and can only use a name. It is because we can't map 1:1 between an image family and self link?

[danawillow]

It's an API thing: https://cloud.google.com/compute/docs/reference/rest/v1/instanceTemplates

Note that for InstanceTemplate, specify the disk name, not the URL for the disk.

(it does seem to accept one for regional disks, but it's not documented so I'm not going to document it on our side)

[ghost]

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