If the NVP cluster is in 'readonly-mode' during a maintenance
window, some NVP operations may raise a Forbidden error. This
is not currently handled correctly, and Neutron server ends up
returning 500. This patch addresses the problem by ensuring
the right error code is returned.
Fixes bug 1204715
Change-Id: Ibd2cac8286a0978a95f9006142c2a405053dfa00