admin: pass --use-feature=2020-resolver to pip

admin: pass --use-feature=2020-resolver to pip

to silence the warning like

follow the suggestion from pip

ERROR: After October 2020 you may experience errors when installing or updating packages. This is because pip will change the way that it resolves dependency conflicts.

We recommend you use --use-feature=2020-resolver to test your packages with the new resolver before it becomes the default.

Fixes: https://tracker.ceph.com/issues/47636
Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit bc00a8feb0d2f4ddf14401e2b551fddff622d1de)