Add 'count' endpoint

It's been suggested (by Brian Rosmaita) that something that might be useful, especially when looking at multi-region deployments, is the ability to quickly get a summary of counts of types.

We don't expose the aggregations API directly (perhaps we should) but even if we did, it would be useful similarly to facets to expose an explicit endpoint to return a summary of accessible result counts in the format:

{"OS::Nova::Server": {"doc_count": 6}, "OS::Neutron::Net": {"doc_count": 2}}

I don't see any reason not to allow queries and the usual index/type restrictions similar to faceting; this will be a shortcut to insert the aggregation and set size=0 on a query, plus formatting the results in a nicer fashion.

This doesn't preclude adding the ability to run arbitrary aggregations; it's a convenience function with a very clear remit (just as facets).

EDIT - in fact, it occurs that this could be part of the facet response since it's very similar, though unfortunately the facet api response is {"resource type": [facets]} which doesn't lend itself well to adding additional info.