Ad-hoc version in bootstrap media

[agracey]

It might be worth looking at if we could put a version (or ManagedVersion) field into the MachineRegistration spec and then have the installer read that field at install time (instead of image creation time).

This would have two effects:

• An admin wouldn't need to reburn and distribute disks to boot with
• The elemental-operator wouldn't need to recreate media on every release of Teal

Combined with #376 and #369, it seem like this would give an efficient system. The issue could be if a user needs to do a reinstall on site so we would also need a way to build the image into the iso when required?