Fix HTTP response for PortNotFound during boot (v3 API)

If a non-existent Neutron port is passed as a network parameter during create
instance, Nova API should return a 404 error. This was fixed for the V2
API in commit 4721408ea861a66af1ad1df5a45c42addcd8a95f (though with a 400).
This applies the corresponding fix to the V3 API.

Fixes bug 1212137

Change-Id: I27dfba636635eb8ac257785b833b240bc439f9ff
Merge "Fix HTTP response for PortNotFound during boot (v3 API)"