CI: Increase job run attempts to 5

Attempts affect pre failures.
This means we can increase stability of jobs by rejecting nodes
that fail pre without failing runs at the same time (unless we
are really unlucky and hit b0rken nodes 5 times in a row).

Change-Id: I17b7f878c742fa8db66f738526855a02ab9f1905

zuul.d/base.yaml

@@ -4,7 +4,7 @@
     pre-run: tests/pre.yml
     run: tests/run.yml
     post-run: tests/post.yml
-    attempts: 1
+    attempts: 5
     timeout: 7200
     required-projects:
       - openstack/kolla-ansible