Use task.set_failed when task is failed

This method was added a long time ago but was never used.

Change-Id: I4127995e97eb6c68bc0ce44a0afbc6bdcdc0d00d
Fixes: bug 1276555
Start refactoring scenario runner

We should remove spaghetti from code and make it safer.

The current implementation has several issues:
1) If something went wrong (which can be the case) we won't cleanup the cloud completely;
2) There is a lot of spaghetti around catching exceptions and preparing the cloud;
3) There will be even more spaghetti when we try to extend current functionality
with (stress exectution/non admin users/another loads and cloud pre configuration);
4) We have a lot of IFs that switch what kind of exectuion we will use.
This should be done via classes and inheritance IMO.

This patch fixes some of the issues mentioned above:
1) Add Context class that allows to create/delete tmp users and tenants;
2) Add Context class that makes all cleanups. For users if they are present
and for admins if they are present;
3) Changed algorithm of deleting resources (try-catch) e.g. if 1 user
wasn't deleted => we should try to continue and delete others;
4) Add some todos;
5) Rewrite ScenarioRunner.run() method to use these context classes;
6) Rewrite UTs accordingly and also use the correct @mock.patch(...) syntax.

Implements: Blueprint scenario-runner-refactoring
Implements: Blueprint unit-tests-refactoring

Change-Id: I5c6c082ed30390bc000a2371f0dfca31994910cb
Use assertIsInstance to test the results of get_engine and get_provider

Replace to TODO(boris-42) in test_engine and test_provider with
assertIsInstance.

Change-Id: I6c486520af15e928d2098a5c10d348b4a16b11da
Mark task as failed if benchmark validation fails

Closes-Bug: #1276826
Change-Id: Idab924ab51cdd30c232b620abf161af40f3e4ac2
Show whether task failed when priting detailed information

Currently 'task detailed' shows status, config and results, but not whether
the task failed.

Change-Id: I9f58ebff0ef6dc289a317c482272403afe73c797
Trello: https://trello.com/c/Qjq5iQqW