Update read_config_drive to use OS_VERSIONS tuple for readers

Updated read_config_drive: removed the unused version kwarg, used the
OS_VERSIONS tuple from the openstack helper to avoid hardcoding
versions.

Added a comment to the tuple in helpers/openstack.py asking for it to
be kept in chronological order.

cloudinit/sources/DataSourceConfigDrive.py

@@ -169,15 +169,17 @@ def get_ds_mode(cfgdrv_ver, ds_cfg=None, user=None):
     return "net"
 
 
-def read_config_drive(source_dir, version=openstack.OS_HAVANA):
+def read_config_drive(source_dir):
-    reader = openstack.ConfigDriveReader(source_dir)
-    finders = [
-        (reader.read_v2, [], {'version': version}),
-        (reader.read_v2, [], {'version': openstack.OS_GRIZZLY}),
-        (reader.read_v2, [], {'version': openstack.OS_FOLSOM}),
-        (reader.read_v1, [], {}),
-    ]
     excps = []
+    finders = []
+    reader = openstack.ConfigDriveReader(source_dir)
+
+    # openstack.OS_VERSIONS is stored in chronological order, so to check the
+    # newest first, use reversed()
+    for version in reversed(openstack.OS_VERSIONS):
+        finders.append((reader.read_v2, [], {'version': version}))
+    finders.append((reader.read_v1, [], {}))
+
     for (functor, args, kwargs) in finders:
         try:
             return functor(*args, **kwargs)

cloudinit/sources/helpers/openstack.py

@@ -48,6 +48,7 @@ OS_LATEST = 'latest'
 OS_FOLSOM = '2012-08-10'
 OS_GRIZZLY = '2013-04-04'
 OS_HAVANA = '2013-10-17'
+# keep this in chronological order by time: add new entries to the end
 OS_VERSIONS = (
     OS_FOLSOM,
     OS_GRIZZLY,