default instance capacity to zero at registration/insertion time

if the first health check fails due to AMQP or celery issues, the
capacity will stay at the default of 100 (which is confusing)

see: https://github.com/ansible/tower/issues/2085

awx/main/managers.py

@@ -96,7 +96,7 @@ class InstanceManager(models.Manager):
             instance = self.filter(hostname=hostname)
             if instance.exists():
                 return (False, instance[0])
-            instance = self.create(uuid=uuid, hostname=hostname)
+            instance = self.create(uuid=uuid, hostname=hostname, capacity=0)
         return (True, instance)
 
     def get_or_register(self):