Cinder count api

Problem description
There is no current API that can retrieve summary count data for volumes and search filters. For example, getting the total number of volumes that match a given filter criteria (for example, all volumes in a given state). Retrieving all volumes and then manually determining the count data does not scale.

Use Case
A UI dashboard that contains volumes in various states. A new API is needed to retrieve the volume count data associated with various filters (ie, volumes in available state, volumes in error state, etc.)