Mattermost with external PostgreSQL

gitlab.rb

mattermost_nginx['redirect_http_to_https'] = true
mattermost_nginx['ssl_certificate'] = "/etc/gitlab/ssl/gitlab.example.com.crt"
mattermost_nginx['ssl_certificate_key'] = "/etc/gitlab/ssl/gitlab.example.com.key"
mattermost['service_use_ssl'] = true

mattermost['email_enable_sign_in_with_username'] = true
mattermost['email_enable_sign_in_with_email'] = false
mattermost['email_enable_sign_up_with_email'] = false
mattermost['email_send_email_notifications'] = true
mattermost['email_connection_security'] = nil
mattermost['email_feedback_name'] = "GitLab Mattermost"
mattermost['email_feedback_email'] = "gitlab@example.com"
mattermost['email_smtp_server'] = "localhost"

mattermost['sql_driver_name'] = 'postgres'
mattermost['sql_data_source'] = "user=mattermost host=/var/run/postgresql port=5432 dbname=mattermost"

mattermost['gitlab_enable'] = true
mattermost['gitlab_id'] = "12345"
mattermost['gitlab_secret'] = "12345"
mattermost['gitlab_scope'] = ""
mattermost['gitlab_auth_endpoint'] = "https://gitlab.example.com/oauth/authorize"
mattermost['gitlab_token_endpoint'] = "https://gitlab.example.com/oauth/token"
mattermost['gitlab_user_api_endpoint'] = "https://gitlab.example.com/api/v3/user"

The DB user is called 'mattermost' (like the system user). The production database for mattermost is called 'mattermost' as well.