cli: add regenerate-repos command
    
    Fix #1915
    See PR#1907
    
    In the PR#1907, @tstellar already implemented the support for
    regenerating repositories in APIv3 (on the frontend side) and in the
    python-copr package. I am just finishing it by adding support for
    copr-cli and covering it with beaker test.
    
    Also, there was two typos in python-copr in the API endpoint URL, so I
    am fixing them as well.