Add itersubclasses() method that allows to iter through all subclass

Add to rally.utils one new method.
This method is required at least to build deploy EngineFactory.

rally/utils.py

@@ -18,6 +18,8 @@
 import StringIO
 import sys
 
+from rally.openstack.common.gettextutils import _   # noqa
+
 
 class StdOutCapture(object):
     def __init__(self):
@@ -41,3 +43,22 @@ class StdErrCapture(object):
 
     def __exit__(self, type, value, traceback):
         sys.stderr = self.stderr
+
+
+def itersubclasses(cls, _seen=None):
+    """Generator over all subclasses of a given class in depth first order."""
+
+    if not isinstance(cls, type):
+        raise TypeError(_('itersubclasses must be called with '
+                          'new-style classes, not %.100r') % cls)
+    _seen = _seen or set()
+    try:
+        subs = cls.__subclasses__()
+    except TypeError:   # fails only when cls is type
+        subs = cls.__subclasses__(cls)
+    for sub in subs:
+        if sub not in _seen:
+            _seen.add(sub)
+            yield sub
+            for sub in itersubclasses(sub, _seen):
+                yield sub

tests/test_utils.py

@@ -46,3 +46,21 @@ class StdIOCaptureTestCase(test.NoDBTestCase):
 
         self.assertEqual(err.getvalue().rstrip('\n').split('\n'), messages)
         self.assertEqual(stderr, sys.stderr)
+
+
+class IterSubclassesTestCase(test.NoDBTestCase):
+
+    def test_itersubclasses(self):
+        class A(object):
+            pass
+
+        class B(A):
+            pass
+
+        class C(A):
+            pass
+
+        class D(C):
+            pass
+
+        self.assertEqual([B, C, D], list(utils.itersubclasses(A)))