BZ#1152543 2 of 2 - use only autodetection for 'Foreman URL'

https://bugzilla.redhat.com/show_bug.cgi?id=1152543

Removes 'Foreman URL' from the configuration wizard and always uses
autodetection (FQDN based) for its value.

Foreman web app needs to be accessible both from the hosts being
provisioned (to fetch their kickstarts and ENCs) and for the user to
interact with the web UI, so it's probably not very beneficial to move
Foreman web app out of machine's FQDN.