Raise correct exception for duplicate networks

When using Quantum, if duplicate network ids are passed in on a
nova boot, nova-api raises a NetworkNotFound exception, which is
not correct.  This change causes it to raise a NetworkDuplicated
exception instead.

Fixes bug 1165088

Change-Id: I4f1f9aa5e1fa0e740b78e67760d924e2dd1d4d98
Merge "Raise correct exception for duplicate networks"