Task helpers partial refactoring added

What is done:
- most methods from TaskHelper class moved to objects.Task;
- code that was dependent on TaskHelper's methods updated
- unit and integration tests updated

What is missed:
- TaskHelper class is still in use as far as it holds several common use
  helper function (in future will be moved to utils module);

Change-Id: Iff2f2a4d277590db25c6f801f20ae75433937411
Implements: blueprint nailgun-task-helpers-refactoring