update_system should be able to send all the data as at announce_system (bnc#889778)
allow to set distro_target durin update_system call
Merge pull request #167 from SUSE/review_140905_allow_to_set_distro_target_on_update_system_call_2183

[Review] Request from 'digitaltom' @ 'SUSE/connect/review_140905_allow_to_set_distro_target_on_update_system_call_2183'