Include a --hooks flag to pass custom install/reset/upgrade hook files.

[davidcassany]

Is your feature request related to a problem? Please describe.

It is hard to automate hooks creation from the reset environment because oem and /usr/local/cloud-config are paths likely to be formatted within the procedure and /system/oem is a readonly path. In addition setting hooks to be automated within the chroot environment is not obvious as default search paths are also chrooted in that process.

Describe the solution you'd like

We would like having a --hooks flag in elemental install/reset/upgrade to pass hooks files directly. Those files would be read before hand in memory and passed to the elemental run-stage accordingly.

Describe alternatives you've considered

Adding extra default path for cloud-config files in /etc/elemental/cloud-config. The problem of this approach is that it does not solve the complexity of passing files to a chrooted hook.

Additional context

This will be helpful to improve rancher/elemental-operator#489 as this could allow to pass system-agent config and registration config and state to the elemental install/reset procedures so there is no need to actually include a cloud-init file to create them at boot for every boot. We could just pass those files to the appropriate location based on a after-install-chroot hook.