Allow system reconfiguration from initrd

[arianvp]

Idea would be to do a nixos-install from the initrd.

This would open up a few new features:

• Repartitioning the disk instead of using a pre formatted ext4 disk. Allows us to fix Configurable boot and swap partitions #112 this is especially important for swap which should be based on the amount of RAM which we don't know apriori
• More reliable than nixos-rebuild switch from User-Data

https://functional.cafe/@arianvp/113149195815755180

Would give something like https://coreos.github.io/ignition/