Merge branch '3060-mattermost-logging-prefix-whitelist' into 'master'

Resolve "Warning noise with deprecated Mattermost svlogd_prefix setting"

Closes #3060

See merge request gitlab-org/omnibus-gitlab!2181

files/gitlab-cookbooks/gitlab/libraries/gitlab_mattermost.rb

@@ -97,6 +97,7 @@ module GitlabMattermost
          env
          host
          port
+         svlogd_prefix
          service_site_url
          service_address
          service_port