Choose precise, centos6, and trusty nodes properly

The zuul node label selection functions missed a few corner cases where
precise, centos6, or trusty needed to be selected. Specifically if the
job name includes precise that job should run on precise. If the job
name says centos6 that job should run on centos6 and so on.

Change-Id: If9a99c0347412c8bb54dd663dbdca4908cb3eb03