Cover Rally with docstrings & Test this coverage

Rally should have detailed docstrings for:
* Benchmark scenario classes
* Benchmark scenarios
* Deploy engines
* Server providers
* SLA

Here we add such docstrings and also add a test suite that checks
that Rally is 100% covered with docstrings and that these docstrings
are correctly formed.

We also change the interface of the following benchmark scenarios
(for the sake of unification):
* CinderVolumes.create_and_attach_volume
* CinderVolumes.create_snapshot_and_attach_volume
* CinderVolumes.create_nested_snapshots_and_attach_volume

Finally, we refactor a bit NovaServers.boot_and_bounce_server.

Change-Id: Ia38c8fc2d692a09719d3e068d332647d4b0da47f