Fix stable branch selector in zuul layout

The branch selector intended to make sure that the -icehouse and
-havana devstack-gate branchless tests only run on master was also
catching the periodic-.*-havana jobs (and icehouse).  Add a negative
match for periodic to that regex.

Change-Id: I35289e80157cf7dac741250932742f75af7b7f55
Merge "Fix stable branch selector in zuul layout"