Fix shared_file_storage_dir on upgrades

If the cluster was upgraded from 2.4 or earlier, this key won't exist
(it's only set to a correct value on cluster init), so we need to
properly set it to a null string (disabled).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>

lib/objects.py

@@ -1169,6 +1169,10 @@ class Cluster(TaggableObject):
     if self.prealloc_wipe_disks is None:
       self.prealloc_wipe_disks = False
 
+    # shared_file_storage_dir added before 2.5
+    if self.shared_file_storage_dir is None:
+      self.shared_file_storage_dir = ""
+
   def ToDict(self):
     """Custom function for cluster.