integration/zuul.git related jobs made concurrent

Two jobs are explicitly serialized:
- integration-docroot-deploy : updates the integration website
- integration-jjb-config-test : has a shared directory :(

Other jobs are made concurrents:
- integration-zuul-layoutdiff
- integration-zuul-layoutvalidation

Change-Id: I6b5f0b85c27d672106629720e634e9d5948adc73