Set zuul version as 2.5.1
Set zuul version as 2.5.1

As the latest 2.6.1.dev version zuul added more dependent packages
which have broken our image building, let's set the zuul version
as 2.5.1 before we migrate the CI env to zuul3.

citrix-openstack/os-ext-testing,citrix-openstack/os-ext-testing