Prepare instances for SSH access

This patchset is in preparation for a scenario for running a
test/benchmark within an instance.

Change-Id: I94276237fb5b453b194e58ed00df589a3420a6ed
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