Convert sandbox_enabled value to string

Environment variable requires string
https://docs.docker.com/compose/compose-file/#args

Change-Id: I831a16b275fbb2431a17c53c4f13f1a393309309
Related-bug: PROD-24686 (PROD:24686)
diff --git a/docker/swarm/stack/monitoring/sf_notifier.yml b/docker/swarm/stack/monitoring/sf_notifier.yml
index a66286a..a171ce8 100644
--- a/docker/swarm/stack/monitoring/sf_notifier.yml
+++ b/docker/swarm/stack/monitoring/sf_notifier.yml
@@ -41,4 +41,4 @@
                 SFDC_PASSWORD: "${_param:sf_notifier_sfdc_password}"
                 SFDC_ORGANIZATION_ID: "${_param:sf_notifier_sfdc_organization_id}"
                 SFDC_ENVIRONMENT_ID: "${_param:sf_notifier_sfdc_environment_id}"
-                SFDC_SANDBOX_ENABLED: ${_param:sf_notifier_sfdc_sandbox_enabled}
+                SFDC_SANDBOX_ENABLED: "${_param:sf_notifier_sfdc_sandbox_enabled}"