Prevent a crash when no entries are successfully migrated.

It would fail in _update_default_group() because migrate_cnt wasn't
defined in context.

https://fedorahosted.org/freeipa/ticket/3386

ipalib/plugins/migration.py

@@ -750,6 +750,7 @@ can use their Kerberos accounts.''')
             valid_gids = []
             invalid_gids = []
             migrate_cnt = 0
+            context['migrate_cnt'] = 0
             for (dn, entry_attrs) in entries:
                 context['migrate_cnt'] = migrate_cnt
                 s = datetime.datetime.now()