fixes nova resize bug when force_config_drive is set.

If files is None, set contents=[] when calling
instance_metadata.InstanceMetadata from within _create_image() to prevent
the method trying to iterate a NoneType.

Fixes bug 1183003

Change-Id: I0ac18ad5363ab9496fa83d231d7ec7c59d737691
Merge "fixes nova resize bug when force_config_drive is set."