Make dnf more robust and faster
    
    Sometimes the prepare-build step of azure pipelines fails
    with download errors:
    "configure: error: Package requirements (nspr) were not met:"
    This can be due to fastestmirror not being used to check
    mirror availability and sometimes speed. Combined with a
    too-low default number of retries, and a high timeout this
    can lead to download failures that could be avoided.
    Activate fastestmirror, add more download workers, and tune
    timeout/retries to make dnf more reliable.
    
    Fixes: https://pagure.io/freeipa/issue/7999
    Signed-off-by: François Cami <fcami@redhat.com>
    Reviewed-By: Christian Heimes <cheimes@redhat.com>