Set the new target distro value at update (bnc#889778) 

- tested in regular SLES11 update via SCC
- @jsrain tested update using a SMT instance

Out of scope of this PR, but just curious: What are the allowed values of target_distro? "SLE12", "SLES12", "SLE12-s390"? http://www.rubydoc.info/github/SUSE/connect/SUSE/Connect/YaST.update_system does not say.