Add pre-network hook and example showing config-then-reboot

Add pre-network hook and example showing config-then-reboot

There are some requirements for early configuration that involves
e.g setting kernel parameters then rebooting.  Currently this can
be done via cloud-init, e.g firstboot templates, but there's been
discussion around enabling a SoftwareDeployment approach instead.

The main advantage of doing it this way is there's an error path
if something goes wrong with the config (except triggering the
reboot as we have to use NO_SIGNAL for that).

Change-Id: Ia54ee654f755631b8062eb5c209a60c6f9161500