Wire up distro selection in instance create to global images

[just-be-dev]

I was talking to @jmpesp about this.

Essentially we want to make sure the distros tab are actual bootable images for the demo. It'll likely only be limited to 1 or 2 images.

This will depend on a distro field being added to GlobalImage such that we have something to key off of. As per the conversation with @jmpesp, we'd also like version to be required in the POST for creating a global image.

[jmpesp]

oxidecomputer/omicron#1092 opened

[david-crespo]

Closed by #923