Benchmark scenarios for Rally: Nova utils

This patch contains the base class for Rally benchmark scenarios for Nova.
The class provides several helper methods for booting/suspending servers
and images. These methods will be of great use in the actual benchmark
scenarios for Nova servers, floating_ips, metadata enrties etc.

Blueprint benchmark-scenarios

Change-Id: I29301d6ee1b1cca2939004acf3d681b01f101b44