user-accounts: Tell realmd not to setup system management

The Enterprise Login feature is meant to setup a individiual
domain accounts for use with GNOME, and not manage the system.
realmd now configures more than just logins, so tell realmd
to hold back and not configure a system managed by the domain.

https://bugzilla.gnome.org/show_bug.cgi?id=697910

https://bugzilla.gnome.org/show_bug.cgi?id=700996