Fix HTTP response for PortInUse during boot (v3 API)

If an in use port is passed as a network parameter in the request, Nova API
should raise an HTTPConflict (409) error. This was fixed in commit
48b31c9a1f5d2a45d881e70c6223411c5abbcb72 for the V2 API but not the V3 API.

Fixes bug 1212098

Change-Id: I94429f2c04cf81cb88a8739f65a49aa7ca0d49bd
Merge "Fix HTTP response for PortInUse during boot (v3 API)"