Validate dscp value less that 64

Previously neutron would raise a 500 error if the dscp value was greater
than 63. This patch adds validation to neutron so that a clear error
message is returned if the value is greater than 63.

Fixes bug 1200026

Change-Id: I41a93661952669d112463608be907eede1901490
Merge "Validate dscp value less that 64"