Enable autostart in osbash

This change enables autostart functionality for osbash.

activate_autostart is called by a minimal script written by
preseed/kickstart files (if configured).

It creates (and grants sudo privileges to) the osbash user if
that has not been done by preseed/kickstart.

It uses template-osbashauto to generate /etc/init.d/osbashauto and
configures the system to run the script immediately and during every
subsequent system boot.

osbashauto iterates over all files in the autostart directory (in the
VirtualBox shared folder; filled by osbash from the host side) and
executes them as the osbash user. It writes its log files into the log
directory in the shared folder.

Partial-Bug: 1312764
Implements: blueprint openstack-training-labs
Change-Id: I0394c5ea4e3f75a9d9bee30d119e047a762e86ce