Allow to specify default provides at task definition

It is now possible to specify what task provides when task class
(deriving from taskflow.task.Task) is defined, using 'default_provides'
class attribute or property. This specification may still be overridden
when task instance is created via 'provides' constructor parameter.

Change-Id: If300c3a35807422d06c0196e84891faebde83c02
Merge "Allow to specify default provides at task definition"