run-selenium: Use latest tag on firefox image

I'm not sure why I used this tag; I probably copied it from [1] at the
time?  Let's just try latest.

Update matchers so the screenshot jobs run

[1] https://github.com/SeleniumHQ/docker-selenium

Change-Id: I8ea7981dac54883822f3b6076b6f0f564571f018

playbooks/roles/run-selenium/tasks/main.yaml

@@ -2,4 +2,4 @@
   shell:
     executable: /bin/sh
     cmd: |
-      docker run -d --network="host" -v /dev/shm:/dev/shm selenium/standalone-firefox:4.0.0-beta-1-prerelease-20201202
+      docker run -d --network="host" -v /dev/shm:/dev/shm selenium/standalone-firefox:latest

zuul.d/system-config-run.yaml

@@ -547,6 +547,7 @@
       - playbooks/roles/codesearch/
       - playbooks/roles/install-docker/
       - playbooks/roles/pip3/
+      - playbooks/roles/run-selenium/
       - docker/hound/
       - testinfra/test_codesearch.py
 
@@ -649,6 +650,7 @@
       # From haproxy-statsd_files -- If we rebuild the image, we want
       # to run this job as well.
       - docker/haproxy-statsd/
+      - playbooks/roles/run-selenium/
 
 - job:
     name: system-config-run-grafana
@@ -677,6 +679,7 @@
       - playbooks/roles/grafana/
       - playbooks/roles/install-docker/
       - playbooks/roles/pip3/
+      - playbooks/roles/run-selenium/
       - testinfra/test_grafana.py
 
 - job:
@@ -798,6 +801,7 @@
       - playbooks/roles/lodgeit
       - playbooks/roles/install-docker/
       - playbooks/roles/pip3/
+      - playbooks/roles/run-selenium/
       - playbooks/test-paste.yaml
       - testinfra/test_paste.py
 
@@ -983,6 +987,7 @@
       - playbooks/rename_repos.yaml
       - inventory/service/group_vars/review.yaml
       - playbooks/roles/pip3/
+      - playbooks/roles/run-selenium/
       - playbooks/roles/install-docker/
       - playbooks/roles/letsencrypt
       - playbooks/roles/gerrit/