Needs verification for *_ workers value

[Liyingjun]

When we start the nova-api service, the *_workers (metadata_workers, ec2_workers, osapi_compute_workers) value are not checked if the value is proper.
If the user passes a negative value, an exception should be threw.

reproduce:
osapi_compute_workers = -1

Here is part of nova-api LOG

2014-01-13 06:51:00.371 INFO nova.wsgi [-] osapi_compute listening on 0.0.0.0:8774
2014-01-13 06:51:00.371 INFO nova.openstack.common.service [-] Starting -1 workers
2014-01-13 06:51:00.372 INFO nova.network.driver [-] Loading network driver 'nova.network.linux_net'
2014-01-13 06:51:00.495 INFO nova.wsgi [-] metadata listening on 0.0.0.0:8775
2014-01-13 06:51:00.496 INFO nova.openstack.common.service [-] Starting 1 workers
2014-01-13 06:51:00.499 INFO nova.openstack.common.service [-] Started child 51886
2014-01-13 06:51:00.501 INFO nova.metadata.wsgi.server [-] (51886) wsgi starting up on http://0.0.0.0:8775/

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/67355

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/67355
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=eba96f363e370ebdbd08c3bdfbe8f57df6cb7a80
Submitter: Jenkins
Branch: master

commit eba96f363e370ebdbd08c3bdfbe8f57df6cb7a80
Author: liyingjun <email address hidden>
Date: Mon Jan 13 07:35:31 2014 +0800

    Workers verification for WSGI service

    When we start the nova-api service, the *_workers (metadata_workers,
    ec2_workers, osapi_compute_workers) value are not verified if the value
    is negative. If the user passes a negative value accidentally, and
    InvalidInput exception will be threw.

    Change-Id: I8337d672cb119efe700aa6be420fdac9864c1577
    Closes-bug: #1269997