allow to pass distro_target to announce_system
Catch zypper error exit status and raise exception

To know if there was a YaST lock we need to look for error code 7
(ZYPPER_EXIT_ZYPP_LOCKED) from zypper.  Next I need to add a DRY way to
handle the various exceptions possible in zypper.rb.
Revert "Catch zypper error exit status and raise exception"

This reverts commit 2b8e66c49b44f19b73a983dbb9ef6a63d07f588c.
Merge pull request #54 from SUSE/review_140414_distro_target

Allow to pass distro_target to announce_system