Fix inconsistent default value type

The share_server_migration_timeout option is an integer option so its
default value should be an integer.

Change-Id: I20a3e8cc8d6b7197d130b5c300f86f5bae2d50db
diff --git a/manila_tempest_tests/config.py b/manila_tempest_tests/config.py
index dc16100..532d2a5 100644
--- a/manila_tempest_tests/config.py
+++ b/manila_tempest_tests/config.py
@@ -322,7 +322,7 @@
                help="Time to wait for share backup before "
                     "timing out (seconds)."),
     cfg.IntOpt("share_server_migration_timeout",
-               default="1500",
+               default=1500,
                help="Time to wait for share server migration before "
                     "timing out (seconds)."),
     cfg.StrOpt("default_share_type_name",