Support multiple plans (along with concurrency / iterations) in a single yaml

[rocketraman]

Sometimes it is helpful to test the behavior of a system under different loads. For example, testing scale up and scale down behavior.

To do this, it is useful to be able to run multiple plans, one after the other.

The workaround for now is to do something like:

drill --benchmark 1 && \
  drill --benchmark 2 && \
  drill --benchmark 3

However, the downside of this approach is that such a test has to be documented externally from the benchmark file.

It would be nice to use the multi-doc feature of yaml to just define the 3 plans in a single benchmark file e.g.:

concurrency: 1
iterations: 100
plan:
  ...
---
concurrency: 10
iterations: 100
plan:
  ...
---
concurrency: 5
iterations: 100
plan:
  ...