Add a SUSE-only crowbar_register utility

This can be used to register a node that was not installed by Crowbar.

This is SUSE-only as support for other OS would probably need some
slight changes to the script to really work. But it should be possible
to do that, provided someone wants to do it.
crowbar_register: Generate addrepo commands with the right URLs

We were hard-coding URLs before, now it should be correct.
crowbar_register: Ask for confirmation before altering the system

There is a new --force argument to skip this step.
Merge pull request #238 from vuntz/crowbar_register

Add a SUSE-only crowbar_register utility