Skip functional tests for documentation update

Functional tests are not needed for non-functional update.

Also skip functional tests for pre-commit config file change which is
not used in functional tests either.

Change-Id: Ife552e6796d97ad24bdeaad76512bf42596b980f

.zuul.yaml

@@ -14,6 +14,12 @@
     pre-run: playbooks/tests/functional/pre.yml
     vars:
       tox_envlist: functional
+    irrelevant-files:
+      - ^.*\.rst$
+      - ^doc/.*$
+      - LICENSE
+      - ^releasenotes/.*$
+      - ^\.pre-commit-config\.yaml$
 
 - job:
     name: oslo.cache-functional-etcd3gw