Add a openstack specific datasource

Openstack has a unique derivative datasource
that is gaining usage. Previously the config
drive datasource provided part of this functionality
as well as the ec2 datasource, but since new
functionality is being added to openstack is
seems benefical to combine the used parts into
one datasource just made for handling openstack
deployments.

This patch factors out the common logic shared
between the config drive and the openstack
metadata datasource and places that in a shared
helper file and then creates a new openstack
datasource that readers from the openstack metadata
service and refactors the config drive datasource
to use this common logic.

cloudinit/util.py

@@ -31,6 +31,7 @@ import glob
 import grp
 import gzip
 import hashlib
+import json
 import os
 import os.path
 import platform
@@ -385,6 +386,15 @@ def multi_log(text, console=True, stderr=True,
             log.log(log_level, text)
 
 
+def load_json(text, root_types=(dict,)):
+    decoded = json.loads(text)
+    if not isinstance(decoded, tuple(root_types)):
+        expected_types = ", ".join([str(t) for t in root_types])
+        raise TypeError("(%s) root types expected, got %s instead"
+                        % (expected_types, type(decoded)))
+    return decoded
+
+
 def is_ipv4(instr):
     """determine if input string is a ipv4 address. return boolean."""
     toks = instr.split('.')