NSX: return 400 if dscp set for trusted queue

If qos_marking is set to True, the code currently silently
drops the dscp setting and continues.

It is preferrable instead to notify the user of the invalid
setting returning a 400 error.

Change-Id: I7db1867712bd20f2030c56a42baac20e6ae76d0d
Closes-Bug: 1332062
(cherry picked from commit f24482b18428a4935cbda97f96532e7c0876d9bd)