give value [] if disallowed_metadata_keys is None

The default value of DiskFile.disallowed_metadata_keys is None. And it
will be iter in following codes even if it didn't get a new value. So
it is necessary to give Diskfile.disallowed_metadata_keys an empty list
in __init__ if the default value None is used.

fixes bug #1206740

Change-Id: I8454eba47b2c691166924356ab33b0235a800951
Merge "give value [] if disallowed_metadata_keys is None"