dconf: Read from the 'gdm' system profile

This enables downstreams to modify GDM settings in a well defined
location without altering the actual GDM defaults that live under
/usr/share/gdm/greeter-dconf-defaults, or mess with the GDM user's
DConf database.

Closes GNOME/gdm#715
1 job for gbsneto/read-system-db-gdm in 4 minutes and 39 seconds (queued for 7 seconds)