zuul: update layout_config path

We have migrated the Zuul configuration repository to
integration/config.git and moved layout.yaml under the /zuul/ sub
directory.

A symbolic link was left over and still used, adjust the Zuul
configuration file to point directly to the file. The symbolic link can
then be removed.

Change-Id: I48f401482bf03a2a59854389256c06068e5ff269