Return policy error, not generic error if nova net-create/delete is forbidden by policy

[Tushar Kalra]

When nova net-create and net-delete are prohibited by policy, we should raise policy violation error (403) to the user instead of service unavailable (503) error which is incorrect.

Steps to reproduce:
1. Add the following policies to policy.json:
"network:create": "rule:admin_api",
"network:delete": "rule:admin_api"

2. As a non-admin user, run nova net-create:
$ nova net-create xyz 192.168.254.1/30
ERROR: Create networks failed (HTTP 503)

Here's the output of other forbidden commands:
$ nova baremetal-node-list
ERROR: Policy doesn't allow compute_extension:baremetal_nodes to be performed. (HTTP 403)

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/62123

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/62123
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=3241155cfac7b8a3306811fc037af72cb2b63590
Submitter: Jenkins
Branch: master

commit 3241155cfac7b8a3306811fc037af72cb2b63590
Author: Tushar Kalra <email address hidden>
Date: Fri Dec 13 17:11:32 2013 -0800

    Return policy error, not generic error

    When nova net-create and net-delete are prohibited by policy, we should
    raise policy violation error (403) to the user instead of service
    unavailable (503) error which is incorrect.

    Closes-Bug: #1260905
    Change-Id: I2edc0eb09f1dc192ba47184ed24fc49e4fcd4b79