beaker, doc: move Install/*/runtest.sh into an image
    
    Being pretty tired of running the Install/Config-copr-cli/runtest.sh
    script all the time in fresh container environment, I finally decided to
    drop it and make the testing container image working out of box.
    
    The purpose of runtest.sh was to "preconfigure" any host (not only the
    container), but we spawn the tests only from containers nowadays.  I
    documented the set of dependencies previously installed by the removed
    script inside README.
    
    Fix the documentation for sanity tests.