Add testresult commentlink

Will take effect when Zuul is running this change:
  I74702fd7d37358e6f4caa7e7ac0a3ede73184077

This change also adds that feature to the Zuul config and enables
it for OpenStack.  It also adds the ability to specify HTML in a
commentlink (and uses it).

Change-Id: Idb4ad8e6079165d681271987a92cab5d8b7c81be
Merge "Add testresult commentlink"