Rename "build_service" directory to "obs"

The new name is more consistent with the constant name used in code and
terminology used in README.md ("OBS directory"). It is also shorter.
Remove "yast" subdirectory from killer's data directory structure

The "yast" directory was the only subdirectory in killer's data
directory and as such it was pretty much useless. Since the data
directory structure is now relatively stable, this state probably won't
cahnge. Let's get rid of the directory.
Merge pull request #384 from yast/directory-changes

[Review] Request from '' @ 'yast/ycp-killer/directory-changes'