capture keystone performance benchmarks

Discussed at the Havana ODS, there's a need to capture and provide keystone performance benchmarks over time.

Options to dig through:
* Tempest is starting and/or talking about doing some of this based on mining log data from devstack gate runs.
* timjr@yahoo is using ZipKin to do whole-system tracing and analysis (https://etherpad.openstack.org/zipkin-tracing)
  - possibly leverage diffs and tomograph library (https://github.com/timjr/tomograph) for extending tracing points
* heckj has a script that mines HAProxy logs for basic performance stats from an external API perspective