Add default case for mysqld_bin

If mysqld_bin isn't found, there are bigger
problems, but the unit tests fail, citing mysqld_bin
being referenced before assignment.

Change-Id: If570c9863ea703fb35ff4faa075130c14a8c8f23

trove/guestagent/datastore/mysql/service.py

@@ -114,6 +114,8 @@ def load_mysqld_options():
         if os.path.isfile(bin):
             mysqld_bin = bin
             break
+    else:
+        return {}
     try:
         out, err = utils.execute(mysqld_bin, "--print-defaults",
                                  run_as_root=True, root_helper="sudo")