slapproxy: Adapt hateoas support to match ERP5

It turns out that ERP5 returns
```
'form_id': {
  'type': 'StringField',
  'key': 'form_id',
  'default': 'InstanceTree_viewAsHateoas',
},
```
for the `form_id` in hateoas, so adapt the proxy to do the same.

slapos/proxy/views.py

@@ -1090,8 +1090,8 @@ def hateoas_partitions(partition_reference):
       '_view': {
         'form_id': {
           'type': 'StringField',
-          'key': 'partition',
-          'default': partition['reference'],
+          'key': 'form_id',
+          'default': 'InstanceTree_viewAsHateoas',
         },
         'my_reference': {
           'type': 'StringField',