Fix creation of trusted queues for the NVP plugin.

Currently if an admin tries to create a trusted queue, Neutron will
fail with MissingDSCPForTrusted exception. However, the NVP semantic
is exactly the opposite, namely the DSCP field must be specified for
untrusted queues and omitted trusted ones.

Fixes bug #1204256

Change-Id: I935fab45fc811a296411283a641b66c5ca96264d
Merge "Fix creation of trusted queues for the NVP plugin."