Pin zuul to 2.6.0 in project-config tests
Pin zuul to 2.6.0 in project-config tests

Zuul master branch is now 3.0.0 which is incompatible with our
environment, so we need to pin to 2.6.0.

CiscoSystems/project-config-third-party,CiscoSystems/project-config-third-party