[Core] Add missed @six.add_metaclass(abc.ABCMeta)

Some core classes define abstract methods that are actually
not abstract enough :)

Change-Id: If030239ea1033bf19e080d139ab7c7447c36bdcf

rally/task/context.py

@@ -52,6 +52,7 @@ def configure(name, order, hidden=False):
 
 
 @plugin.base()
+@six.add_metaclass(abc.ABCMeta)
 class Context(plugin.Plugin, functional.FunctionalMixin,
               utils.RandomNameGeneratorMixin):
     """This class is a factory for context classes.

rally/task/runner.py

@@ -20,6 +20,7 @@ import multiprocessing
 import time
 
 import jsonschema
+import six
 
 from rally.common import logging
 from rally.common.plugin import plugin
@@ -99,6 +100,7 @@ def _log_worker_info(**info):
 
 
 @plugin.base()
+@six.add_metaclass(abc.ABCMeta)
 class ScenarioRunner(plugin.Plugin):
     """Base class for all scenario runners.
 

rally/task/types.py

@@ -18,6 +18,8 @@ import copy
 import operator
 import re
 
+import six
+
 from rally.common.plugin import plugin
 from rally import exceptions
 from rally import osclients
@@ -99,6 +101,7 @@ def preprocess(name, context, args):
 
 
 @plugin.base()
+@six.add_metaclass(abc.ABCMeta)
 class ResourceType(plugin.Plugin):
 
     @classmethod