artifacts: use requests session for download
artifacts: use requests session for download

Prior to this change, we would throw away the persistent requests
session instead of using it. This meant that even if callers passed in a
persistent requests session, we would just create a new one with
requests.get().

This is a fix for the feature introduced in
6567a1f37d475e98bb63512a723c6fdbcaef0027

red-hat-storage/rhcephcompose,red-hat-storage/rhcephcompose