daemon: Specify DCONF_PROFILE for initial-setup
There have been some recent requests to make some adjustments to the initial-setup session.
I'm not too keen on adding special-cases all over gnome-shell, so the least intrusive option seems a dconf override similar to the one shipped by gdm.
However https://bugzilla.gnome.org/show_bug.cgi?id=780866 looks like it will regress when setting a DCONF_PROFILE again (even if it's a dedicated one instead of re-using gdm).
So I'm putting this up for discussion before spending time on a proper commit message :-)