Added/cleaned request/response parameters for subnetpools rest_methods:
GET /v2.0/subnetpools
GET /v2.0/subnetpools/{subnetpool_id}
PUT /v2.0/subnetpools/{subnetpool_id}
POST /v2.0/subnetpools
DELETE /v2.0/subnetpools/{subnetpool_id}
Change-Id: I03234589b7fb9f0b5b5784ea2419bc380d85e5aa
Partially-Implements: blueprint neutron-in-tree-api-ref