Replaced the ScenarioGetter with the GroundMotionFieldGetter

As discussed in https://bugs.launchpad.net/oq-engine/+bug/1361533 and in https://github.com/gem/oq-risklib/pull/75, the hazard getters for the scenario risk calculators are fragile: any refactoring will break the association between the epsilons and the ground motion value. The GroundMotionFieldGetter has no such problem, since the association between ground motion fields and ruptures is explicit. Therefore we can make the engine more robust by replacing the ScenarioGetter with the GroundMotionFieldGetter; this has also the added benefit of unifying completely the table structure for both event based and scenario calculators.
In this PR I also remove a few obsolete scenario tests, now useless since the QA tests are fast and cover more.
Notice that I had to change slightly the numbers in risk/scenario/case_2/test.py, since now the association
GMFs<->epsilons is different (and more correct).
The tests are running here: https://ci.openquake.org/job/zdevel_oq-engine/590