This patch makes the download_image() to retry automatically on
ContentTooShortError exception, this is an issue that has been seen on
the gate, see [0].

[0] https://openstack-ci-reports.ubuntu.com/artifacts/3ae/891712/3/check/focal-wallaby-pacemaker-remote-ssl_masakari/3ae840c/job-output.txt

(cherry picked from commit 91c1cc3)

Replace "assert type(...) is Foo" with "self.assertIsInstance(..., Foo)"
this produces a more meaningful test error message and fixes pep8 rule
E721[0]

[0] https://www.flake8rules.com/rules/E721.html

(cherry picked from commit f9a4c92)