Refactor: moved flatten function to utils

Previously, the keystone.common.authorization module contained the function
"flatten", which would transform a dictionary (possibly nested) into a
single level dictionary. Given that the "flattening" of a dictionary is
a more general concept, not related to authorization, I thought it would
be more intuitive to have it in the keystone.common.utils module.

Change-Id: Ic1f6fa70634ade41e392ee7c5cd8adac913fd0b8

keystone/common/utils.py

@@ -17,6 +17,7 @@
 #    under the License.
 
 import calendar
+import collections
 import grp
 import hashlib
 import json
@@ -40,6 +41,23 @@ CONF = config.CONF
 LOG = log.getLogger(__name__)
 
 
+def flatten_dict(d, parent_key=''):
+    """Flatten a nested dictionary
+
+    Converts a dictionary with nested values to a single level flat
+    dictionary, with dotted notation for each key.
+
+    """
+    items = []
+    for k, v in d.items():
+        new_key = parent_key + '.' + k if parent_key else k
+        if isinstance(v, collections.MutableMapping):
+            items.extend(flatten_dict(v, new_key).items())
+        else:
+            items.append((new_key, v))
+    return dict(items)
+
+
 def read_cached_file(filename, cache_info, reload_func=None):
     """Read from a file if it has been modified.