Fix the cidr issue

Edouard, neutron core is validating cidr before calling the plugin create_subnet API.
Do you think we still need to handle in the plugin ?

2014-06-25 08:35:37.098 DEBUG neutron.api.v2.attributes [req-3ed2beb0-057d-4d9a-a10a-0b9c4923a01f demo 211d8662d6e24741949009525f08ced5] '28.0.0.0/123' is not a valid IP subnet from (pid=8470) _validate_subnet /opt/stack/neutron/neutron/api/v2/attributes.py:308
2014-06-25 08:35:37.100 ERROR neutron.api.v2.resource [req-3ed2beb0-057d-4d9a-a10a-0b9c4923a01f demo 211d8662d6e24741949009525f08ced5] create failed
2014-06-25 08:35:37.100 TRACE neutron.api.v2.resource Traceback (most recent call last):
2014-06-25 08:35:37.100 TRACE neutron.api.v2.resource   File "/opt/stack/neutron/neutron/api/v2/resource.py", line 87, in resource
2014-06-25 08:35:37.100 TRACE neutron.api.v2.resource     result = method(request=request, **args)
2014-06-25 08:35:37.100 TRACE neutron.api.v2.resource   File "/opt/stack/neutron/neutron/api/v2/base.py", line 378, in create
2014-06-25 08:35:37.100 TRACE neutron.api.v2.resource     allow_bulk=self._allow_bulk)
2014-06-25 08:35:37.100 TRACE neutron.api.v2.resource   File "/opt/stack/neutron/neutron/api/v2/base.py", line 635, in prepare_request_body
2014-06-25 08:35:37.100 TRACE neutron.api.v2.resource     raise webob.exc.HTTPBadRequest(msg)
2014-06-25 08:35:37.100 TRACE neutron.api.v2.resource HTTPBadRequest: Invalid input for cidr. Reason: '28.0.0.0/123' is not a valid IP subnet.
2014-06-25 08:35:37.100 TRACE neutron.api.v2.resource