First up we need python2.7 specific msrest version as azure-storage-blob
depends on it and it doesn't properly specify python versions for its
releases.
Next we cannot use zuul/zuul to test ensure-pip can install packages
because zuul/zuul requires python3.8 or newer now. Some platforms we run
ensure-pip on still only have python3.6. We address this by using
zuul-client instead.
Change-Id: I3d91fd3f85e3d7845ab2d7615a1ebfb614064573