Amend tempest-irrelevant-files to avoid false positives

Both setup.cfg and files in etc/ can be relevant for devstack based
runs, as the recent breakage [0] has shown. Don't ignore them when
deciding whether to run tempest-based jobs.

[0] https://review.opendev.org/707759

Change-Id: I19c4e7d8dba4f9f8f0f3efa2a37245a7510cd052

.zuul.yaml

@@ -244,9 +244,7 @@
               - ^glance/locale/.*$
               - ^glance/tests/.*$
               - ^doc/.*$
-              - ^etc/.*$
               - ^releasenotes/.*$
-              - ^setup.cfg$
               - ^tools/.*$
               - ^tox.ini$
               - ^\.zuul\.yaml$