Don't offline after periodic-mirror-python26

* modules/openstack_project/files/zuul/layout.yaml: A too-greedy
pattern for matching Python 2.6 unit test job names inadvertently
also included the periodic-mirror-python26 job. The upshot was that
Zuul instructed Jenkins to offline the static mirror26 slave where
it runs after job completion. Add a negative lookahead to exclude
this job from the pattern for single-use behavior.

Change-Id: I31852ddfefb630044850af7eeb08627a709411cc