404 periodic job : use executor only, add host key

This only needs to run on the executor, specify a blank nodeset.

Add the static.opendev.org host key after adding the host.

Change-Id: Iedde486ce8f3e9b415991830121fb87ba192afc6

.zuul.yaml

@@ -1158,6 +1158,8 @@
     name: system-config-static-404-checker
     description: |
       Run 404 scraping script on static.opendev.org
+    nodeset:
+      nodes: []
     run: playbooks/periodic/404.yaml
 
 - project:

playbooks/periodic/404.yaml

@@ -8,6 +8,11 @@
         ansible_port: 22
         ansible_user: zuul
 
+    - name: Add static.opendev.org host key
+      known_hosts:
+        hostname: static.opendev.org
+        key: static.opendev.org,23.253.245.150,2001:4800:7818:101:be76:4eff:fe04:7c28 ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBMu3PnnkNhPS2d5Z2uPju3Qqcbbc0lwHA1j9MgHlLnbK3bx1O2Kfez6RJUGl2i6nshdzkKwPBvN2vehQKiw1oSk=
+
 # NOTE(ianw): 2020-02-25 just for initial testing run this for one log
 # in a dumb way.  We can scrape a few more sites.  Overall, we expect
 # this to be replaced with a better analysis tool, see