qa: fix modify primary test

This test uses a non-existing config value "file-storage-dir" and fails.
Fixing it by using the default value, which is currently what qa does.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Bernardo Dal Seno <bdalseno@google.com>

qa/qa_instance.py

@@ -611,7 +611,9 @@ def TestInstanceModifyPrimaryAndBack(instance, currentnode, othernode):
   current = currentnode.primary
   other = othernode.primary
 
-  filestorage = qa_config.get("file-storage-dir")
+  # FIXME: the qa doesn't have a customizable file storage dir parameter. As
+  # such for now we use the default.
+  filestorage = pathutils.DEFAULT_FILE_STORAGE_DIR
   disk = os.path.join(filestorage, name)
 
   AssertCommand(["gnt-instance", "modify", "--new-primary=%s" % other, name],