Fix v2 requests to non-bleeding edge servers

In the case where v2 requests are sent to a server which is not running
head of tree which includes the v2 metadef code some 404 cases need to
be handled to enable standard requests to complete.

This patch aslo improves fetching schemas -- they are now only
fetched as needed.

Change-Id: I8c871f11b909337bd7df19b77e606772dbc634b2
Closes-bug: #1367326

tests/utils.py

@@ -18,6 +18,8 @@ import json
 import six
 import testtools
 
+from glanceclient.v2.schemas import Schema
+
 
 class FakeAPI(object):
     def __init__(self, fixtures):
@@ -60,6 +62,15 @@ class FakeAPI(object):
         return self._request('HEAD', *args, **kwargs)
 
 
+class FakeSchemaAPI(FakeAPI):
+    def __init__(cls, *args):
+        super(FakeSchemaAPI, cls).__init__(*args)
+
+    def get(self, *args, **kwargs):
+        _, raw_schema = self._request('GET', *args, **kwargs)
+        return Schema(raw_schema)
+
+
 class RawRequest(object):
     def __init__(self, headers, body=None,
                  version=1.0, status=200, reason="Ok"):