Add API calls to get object counts

For monitoring and scale management purposes it is important to be able to control the count of objects / resources in the cloud. Right now the only way to do this is to request all objects, which is an overkill for the task (and creates a scalability issue depending on polling interval). The proposal is to add smth/count URL which would return the number of (non-deleted) objects, which would otherwise be returned via /ports or similar requests. This would allow lightweight scale monitoring for Neutron.

The same blueprint makes sense for all other core OpenStack components.