auto-installer: move ssh keys setup to low-level installer

auto-installer: move ssh keys setup to low-level installer

.. thereby, also fixing a accidental shell injection.

Since run_cmd{,s}() is nowhere else used anymore, they can be removed
too.

Also mostly reverts commit

  5878dc4ae "auto-installer: handle auto-reboot info messages directly"

in the process too.

Reported-by: Friedrich Weber <f.weber@proxmox.com>
Signed-off-by: Christoph Heiss <c.heiss@proxmox.com>