add ubuntu-init-switch module for testing systemd.

The module is useful primarily for testing in Ubuntu's transition to systemd.
It should be very harmless elsewhere as it defaults to doing nothing,
and will only run if configured as 'ubuntu' distro *and* 'dpkg' is available.
ubuntu-init-switch: fix for determining if systemd

/run/systemd/ seems to exist even if systemd is not the init system.
but /run/systemd/system/ only exists if it has been used as init.
ubuntu-init-switch: support disabling of reboot, minor tweeks.

- modify timeout for ~60 seconds before going on.
- allow specifying 'reboot' on 'cloud-init single' or in config.
- use stdin for subprocess so log is less noisy.
- reboot --force: this is required this early in boot for upstart
  and to my experience at this point always required for systemd