Based on the ubuntu1804 test image, with the following changes:
- update packages list
- avoid failing gnome session start due to missing gsd-power, which
requires logind
- remove /run/systemd/seats (created at systemd package install), to
avoid gnome-shell thinking it needs to talk to logind
- stop installing node via tooltool, it should be grabbed from the
toolchain task when/if necessary
- stop installing psutil via pip, we can use the ubuntu package instead
- remove setcap of gnome-keyring-daemon binary; it's no longer installed
with the IPC_LOCK capability in newer distro versions
- remove /etc/xdg/autostart/gnome-software-service.desktop (unused)
Differential Revision: https://phabricator.services.mozilla.com/D226477
f7d1ec412d