Copy-files

[methuselah-0]

Would be nice to add a way to have files or directories copied to new VM's, so impöementation-wise would need to read an env var with key-values for source-path -> target-path (ganeti server to VM disk). This could be used to add private (non-world readable) files to new VM's at creation time. An example would be to add private key files for VPN's with some specified permissions. This should run after guix init to allow all folders to have been created first. One option would be to have the env var be expecting json content to allow multiple files with different target permissions.

[methuselah-0]

It would also allow a way to achieve the same functionality as was previously done here: f566bcf

Thoughts?