installer: T7102: download upgrade images to a random path in the root filesystem

[dmbaturin]

Change summary

As of now, the image upgrade script always downloads the new image to /tmp/vyos_installation.iso. There are two problems with that approach:

1. /tmp is a RAM-backed tmpfs, which means upgrade can fail if the system doesn't have enough free RAM to store a ~600MB image. Even if it does have that memory, a completely avoidable 600M RAM usage spike still isn't a good idea.
2. If two people try to execute add system image at once, using a fixed location will lead to funny errors.

Storing the temporary image in the persistent root filesystem and using a random name for it solves both problems.

This PR is somewhat ugly — that's the price of making it minimal and easy to backport. The script can certainly benefit from a deep refactoring.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

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