Add ubuntu-desktop-minimal dep to gnome-config role.

I decided to base my installs on the Ubuntu server image rather than the
desktop version because, going forward, it offers more possibilities for
automation. But this means I can't configure my Gnome setup without
first installing the desktop (which isn't included in the server ISO).

roles/gnomeconfig/tasks/main.yml

@@ -6,6 +6,7 @@
       - dconf-cli
       - gnome-shell-extensions
       - python3-psutil
+      - ubuntu-desktop-minimal
       - unzip
       - zsh