Fix inconsistency between Nova-Net and Neutron

The neutron client raises NeutronClientException (status_code 404)
that is not unknown for nova, and is translated in 500 computeFault.
Catching this exception in nova/nova/network/neutronv2/api.py and
raising more generic exception as exception.NotFound solves the problem.

Added also an unitary test to test neutronv2 api

bug #1160309
bug #1200175

Change-Id: Ib2f1a0d49a12114b28f0cd5ce0bb7119f864d1a6
Merge "Fix inconsistency between Nova-Net and Neutron"