[docs] Fix plugin references for class-based scenarios

InfoMixin obtains documentation from objects __doc__ properties.
In case of class based scenarios, __doc__ property is empty (
we decided to use docstrings of "run" method).

This patch adds new object property - _doc_source_, which should include
object from which docstings will be used.

Change-Id: I799222fe213b96a88110d6d2ebb5aa3fe2d8f153

rally/common/plugin/info.py

@@ -83,7 +83,8 @@ class InfoMixin(object):
     @classmethod
     def get_info(cls):
         plugin_ = getattr(cls, "func_ref", cls)
-        doc = parse_docstring(plugin_.__doc__)
+        doc_source = getattr(plugin_, "_doc_source_", plugin_)
+        doc = parse_docstring(doc_source.__doc__)
 
         return {
             "name": plugin_.get_name(),

rally/task/scenario.py

@@ -50,6 +50,9 @@ def configure(name=None, namespace="default", context=None):
         scen.is_classbased = hasattr(scen, "run") and callable(scen.run)
         if not scen.is_classbased:
             plugin.from_func(Scenario)(scen)
+        else:
+            scen._doc_source_ = scen.run
+
         scen._meta_init()
         if name:
             if "." not in name.strip("."):