only run commands against nodes in runtime layers

Author: Gabriel Monroy

api/models.py

@@ -516,7 +516,7 @@ def logs(self):
 
     def run(self, command):
         """Run a one-off command in an ephemeral app container."""
-        nodes = self.formation.node_set.order_by('?')
+        nodes = self.formation.node_set.filter(layer__runtime=True).order_by('?')
         if not nodes:
             raise EnvironmentError('No nodes available to run command')
         app_id, node = self.id, nodes[0]