BZ#1152543 1 of 2 - fix error message printing

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

Don't swallow exceptions in provisioning_seeder and fix an argument
extraction in `show!` method to get better error messages when a
Foreman host cannot be found in Foreman.
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.