Consume slowlogs from redis in k8s

In our VM setup we consume redis slowlogs through fluentd (see https://gitlab.com/gitlab-cookbooks/gitlab_fluentd/-/blob/a6a1bb6eb35b75896f02ab009393867868f980ed/templates/default/redis.conf.erb#L35-54). We should do the same for our k8s setup.

• Add redis-slowlog plugin to our fluentd-docker image: fluentd-docker!20