Returns 503 if the NVP cluster is in maintenance mode

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
Merge "Returns 503 if the NVP cluster is in maintenance mode"