Both GDM and GNOME uses DConf to manage its own configuration. Extract
the common dependencies into a separate role called "dconf".
DConf provides:
- creating empty directories for configuration
- handler to apply configuration when configuration has changed
Add ability to install GDM as a graphical login prompt.
Also certain global settings can be changed by Ansible variables. See
"gdm" key of group-vars/all file.
