The test test_password_prompted() was expecting a keystone failure, but
because of an error in the requests/urllib3 library it was returning a
different exception and causing our tests to fail, see[1].
Independent of the the problem above on tests we should not rely on some
external library failing like that, we should mock this failures to make
our tests more reliable and that's what this patch is doing.
[1] https://bugs.launchpad.net/ospurge/+bug/1503768
Change-Id: I3e6091eb2bc221a16fdc18b998ba138058818894
Closes-Bug: #1504197