Change Pools to be able to clear healthMonitor property

This resolves MN-2375.

Tested manually. 503 is OK because the request was made correctly. The mapping between the health monitor and the pool is being processed (PENDING_CREATE) should be resolved when the connection between the health monitor and HA proxy is established.

```bash
midonet> list router
midonet> health-monitor create delay 100 max-retries 50 timeout 500 type TCP
hm0
midonet> load-balancer create 
lb0
midonet> load-balancer lb0 create pool 
health-monitor  id  lb-method  load-balancer  protocol  state
midonet> load-balancer lb0 create pool 
health-monitor  id  lb-method  load-balancer  protocol  state
midonet> load-balancer lb0 create pool lb-method ROUND_ROBIN protocol TCP 
lb0:pool0
midonet> load-balancer lb0 pool pool0 set health-monitor hm0
midonet> load-balancer lb0 pool pool0 clear health-monitor 
Internal error: {"message":"The mapping between the health monitor and the pool is being processed (PENDING_CREATE).","code":503}
```