t/a-buildvm-ubuntu-cloud: install 'ubuntu-minimal' and autoremove extra packages

Without installing 'ubuntu-minimal', the dpkg database is left in a state where a bunch of packages like iproute2 or netplan.io can be autoremoved, which will happen if subsequent test on that image use -U, further breaking the testbed if a reboot is triggered after that (like not networking at all). The --autoremove option to the purge cloud-init will ensure that no leftovers are kept, like a bunch a Python modules that cloud-init pulls.

On Trusty with the apt-get purge --autoremove, the following error shows: E: Command line option --autoremove is not understood The %(autoremove)s workaround unfortunately has the limitation of keeping a few extra packages pulled by cloud-init on Trusty images, like software-properties-common, but has the advantage of being quite simple. Once Trusty is truly EOL, the workaround can be removed, because every release up to Xenial support the flag.