Zuul layout.yaml got moved to /zuul/ subdir

Has been done to prepare the merge of the repositories
integration/zuul-config and integration/jenkins-job-builder-config

There is a symbolic link left behind but lets rename it now rather than
being sorry later on.

zuul-config change is:

    https://gerrit.wikimedia.org/r/#/c/165745/

Change-Id: I9225257261c9d5cac6338f249f3ddbb0d91810c8