blockerbugs: define BLOCKERBUGS_URL and simplify the config file
    
    BLOCKERBUGS_URL needs to be defined in order to show correct links in the
    staging environment. While fixing that, realized that most of the if-else
    clauses were unnecessary, because there are already group vars for that. So
    simplified the config file by using the vars. KOJI_URL is dropped because it's
    no longer needed.
    
    Related: https://pagure.io/fedora-qa/blockerbugs/issue/192