Don't set up source and debuginfo repos on clients when not mirrored

[digitaltom]

Registered clients always get the default setup of repositories via the zypper service from RMT.
Typically, this includes disabled source and debuginfo repositories.
When RMT hasn't mirrored those, it would be better not to include them in the service, because clients cannot
use them anyway.

Enhancement request originated from https://bugzilla.suse.com/show_bug.cgi?id=1219883