grafana: update to oss latest release

It seems the images have split into enterprise and oss releases.  The
OSS release is the one we want.

Since If0d584f848f213aeea385885e3decfaee6303de5 we don't run anything
in the container.  So we can switch to the upstream default of the
Alpine-based container, which is their reccommendation.

Just use the :latest tag.  Generally the API seems pretty stable.  If
we do break, it's better to figure it out quickly rather than pin to
an old version and then have to make huge jumps.

Depends-On: https://review.opendev.org/c/opendev/grafyaml/+/825990
Change-Id: I312a141baf73d750591957197cb5ba829f503fcb

playbooks/roles/grafana/templates/docker-compose.yaml.j2

@@ -5,7 +5,7 @@ version: '2'
 services:
   grafana:
     restart: always
-    image: docker.io/grafana/grafana:7.4.2-ubuntu
+    image: docker.io/grafana/grafana-oss:latest
     network_mode: host
     environment:
       GF_SECURITY_ADMIN_PASSWORD__FILE: '/etc/grafana/secrets/admin_password'