[provisioner] Add autoyast self update option

[nicolasbock]

AutoYaST can update itself before installing the OS. This patch enables this feature and adds a field to the provisioner barclamp so that a repository URL can be set by the user. The special token <ADMINWEB> resolves to the IP:port of the admin node. An empty URL defaults to the SUSE customer center (scc.suse.com).