remove default=None for config options

In the cfg module default=None is set as the default value.

Change-Id: I9303e2ae4b8f301757744efc09136868db29472a
Closes-bug: #1323975
diff --git a/manila_tempest_tests/config.py b/manila_tempest_tests/config.py
index 8873ef6..22a2e3c 100644
--- a/manila_tempest_tests/config.py
+++ b/manila_tempest_tests/config.py
@@ -176,6 +176,5 @@
                help="Time to wait for share migration before "
                     "timing out (seconds)."),
     cfg.StrOpt("default_share_type_name",
-               default=None,
                help="Default share type name to use in tempest tests."),
 ]