failed to backup data to ssh storage

When backup data to ssh storage for the first time, because the sub
directory "metadata" isn't exist in the ssh storage directory, the
exception IOError will be raised.

The exception IOError should be caught in the funtion listdir() of the
SshStorage class, and the empty list should be returned when the errno
is ENOENT which indicates "No such file or directory". The other
exceptions, e.g. "Permission denied", should be raised.

Change-Id: Icddca7078a47fb82c12d4232065d51e2baa0ab60
Closes-Bug: #1638816

freezer/storage/ssh.py

@@ -15,6 +15,7 @@ limitations under the License.
 
 """
 
+import errno
 import os
 import stat
 
@@ -98,7 +99,13 @@ class SshStorage(fslike.FsLikeStorage):
         self.ftp.put(from_path, to_path)
 
     def listdir(self, directory):
-        return self.ftp.listdir(directory)
+        try:
+            return self.ftp.listdir(directory)
+        except IOError as e:
+            if e.errno == errno.ENOENT:
+                return list()
+            else:
+                raise
 
     def open(self, filename, mode):
         return self.ftp.open(filename, mode=mode)