Parameterize value `enabled` for cloudwatch-api

Reason: Heat CloudWatch API has been removed from
openstack Queens and above.
Stacklight team have fired Alert about heat cloudwatch but
its a false positive since in recent heat this cloudwatch
is not longer present.

Change-Id: Id540c386f87279d4601b13510b4a548e0f20721f
Related-Prod: PROD-25742 (PROD:25742)
diff --git a/nginx/server/proxy/openstack/heat_cloudwatch.yml b/nginx/server/proxy/openstack/heat_cloudwatch.yml
index 3b2dad6..bea6715 100644
--- a/nginx/server/proxy/openstack/heat_cloudwatch.yml
+++ b/nginx/server/proxy/openstack/heat_cloudwatch.yml
@@ -9,7 +9,7 @@
       enabled: true
       site:
         nginx_proxy_openstack_api_heat_cloudwatch:
-          enabled: true
+          enabled: ${_param:openstack_heat_cloudwatch_api_enabled}
           type: nginx_proxy
           name: openstack_api_heat_cloudwatch
           proxy: