Pin Grafana to amd64 since renderer requires it.

This could be mitigated by moving the renderer to another task group.

core/grafana.nomad

@@ -281,6 +281,11 @@ ${file(join("/", [module_path, "grafana", config_file]))}
     task "grafana-image-renderer" {
       driver = "docker"
 
+      constraint {
+        attribute = "$${attr.cpu.arch}"
+        value = "amd64"
+      }
+
       config {
         image = "grafana/grafana-image-renderer:3.6.1"
         ports = ["renderer"]