Fix inconsistencies in docs for factory classes

Fix several issues in rally.deploy.engine.EngineFactory and
rally.deploy.serverprovider.provider.ProviderFactory:

* Wrong API descriptions
* Wrong module references
* Grammar

Change-Id: Idddf3da0c935ba7d21132c67be9259a1fbf96130
Merge "Fix inconsistencies in docs for factory classes"