Benchmark Cloud Cleanup

After 1 or more benchmarks are run as part of rally, the rally framework should handle cleaning up after itself. This includes:
- Deleting any VMs the benchmark(s) created
- Deleting any images/snapshots the benchmark(s) created
- Deleting any other virtual resources (in the future vols, vol-types, networks, etc)
- Deleting any users/project (tenants) that were created as part of the benchmark(s)
The result should be a Cloud which contains the same resources it did prior to the rally benchmarks.
This logic should be "generic" such that it can be run across all benchmarks when they complete in the rally workflow/lifecycle.