copr-fe: make copr-frontend-upgrade.yml more usable
    
    Without shutting httpd down, sometimes - when there's high enough load -
    post scriptlet of copr-frontend takes too long to finish (restarging
    httpd, gently waiting for existing connections).  But that makes the
    outage window larger as httpd doesn't accept the new connections when
    shutting down.
    
    So automatize (forced) shutdown of httpd first (when there's existing
    frontend update) -> upgrade the package -> run the migrations -> and
    make httpd started again.