qa/tasks/cephfs_test_runner: setattr to class not instance

qa/tasks/cephfs_test_runner: setattr to class not instance

before this change, `setattr()` sets the instance specialized with a certain method
of test case, so in `MgrTestCase.setUpClass()`

assert cls.mgr_cluster is not None

fails,

after this change, instead of test case, the class of test suite is updated with the
specified params, even if we pass a certain test to test runner.
so we can

./run-backend-api-tests.sh tasks.mgr.test_dashboard.TestDashboard.test_standby

now. before this change, we can only:

./run-backend-api-tests.sh tasks.mgr.test_dashboard.TestDashboard

Signed-off-by: Kefu Chai <kchai@redhat.com>