Benchmark scenarios for Rally: run task in instance

This patch allows a user to define a scenario where Nova instances are
created, then a script which outputs arbitrary JSON is run, and then the
instance is destroyed.

This allows tests/benchmarks from the instance perspective, as well as
from OS APIs.

blueprint benchmark-scenarios

Change-Id: I4b1f77f5f8c55fe1da41b6ab0b23eb03303ba95f
Print task information when creating task from CLI

Summary line is printed when task has been created but not started,
detailed information is printed once the task has completed.

Change-Id: I8a3cbb81d7fccffe158517d3023ad47d28d7ebc1
Trello: https://trello.com/c/wJbH8DqG