Test if the file repomd.xml exists rather than just dir repodata

fixes fedora-infra#908
Workaround candidate for fedora-infra#887

bodhi/server/metadata.py

@@ -74,7 +74,7 @@ def __init__(self, release, request, db, path):
         # Load from the cache if it exists
         self.cached_repodata = os.path.join(self.repo, '..', self.tag +
                                             '.repocache', 'repodata/')
-        if os.path.isdir(self.cached_repodata):
+        if os.path.isfile(os.path.join(self.cached_repodata, 'repomd.xml')):
             log.info('Loading cached updateinfo.xml')
             self._load_cached_updateinfo()
         else: