* Remove semantic validation mechanism for context. We will use same
approach as for current scenarios (via decorators)
* Pass "deployment" instead of "tasks" to validators. This allow us to
remove access to DB from validators + we can call validation without
creating tasks
* Add hacking rule to force using construction "from rally import objects"
* Fix unit tests
Change-Id: Ibf08183b28479cc49ab6adfd12298612f6b4397f