In before, we switched from 'auth_uri' to 'www_authenticate_uri' [1]
in a backward-incompatible way, which breaks the legacy setup.
For example, if users install Zun with Pike or earlier version
of OpenStack, it will break.
This patch fixes it by fall back to the legacy option 'auth_uri'
if 'www_authenticate_uri' is not presented. In addition, a unnecessary
import_opt statement is removed.
[1] https://review.openstack.org/#/c/568457/
Change-Id: I5e3535f8dbfac73e56190442a31943ac22562ffb
cc0f66218f