Refactor function child_copy_repo

Make function more compact. It now contains only six statements, compared to
nine previously. Make function `main` only call `child_copy_repo` if `REPO_FILE`
exists. Example output where `REPO_FILE` exists ("…" stands in for the host
name):

    >>> from katello_rapidnode import child_copy_repo
    >>> child_copy_repo('…')
    Copying myrepofile.repo to …...
    >>>