Cannot set some attributes to default values after setting

[yong sheng gong]

Helo,
We cannot set some attributes into default or none after setting it.

For example,
we can run quantum subnet-create net1 10.0.1.0/24 --dns_nameservers server1 server2

but we will not be able to clear it.
This bug is related to our validation logics and CLI command.

on server side: our validation logic is demanding some thing to validate, they don't think None is validate value.
On client command side, we is not able to specify the None value for an attribute.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/17436

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/17441

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/17441
Committed: http://github.com/openstack/quantum/commit/4a6ae51eab09c1c38a52404fe816baad1c67376d
Submitter: Jenkins
Branch: master

commit 4a6ae51eab09c1c38a52404fe816baad1c67376d
Author: gongysh <email address hidden>
Date: Tue Dec 4 14:46:25 2012 +0800

    Support to reset dnsname_servers and host_routes to empty

    Bug #1086232

    Change-Id: Ia27170b206f755577c8607c5b27dc5cf57d19633

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/17436
Committed: http://github.com/openstack/python-neutronclient/commit/f9ca01c81e246d4d596b12a529cb8c56b9e31cb9
Submitter: Jenkins
Branch: master

commit f9ca01c81e246d4d596b12a529cb8c56b9e31cb9
Author: Yong Sheng Gong <email address hidden>
Date: Sun Jul 7 21:55:19 2013 +0800

    Allow user to specify None value to attributes

    Bug #1086232

    We introduce the action=clear to unknown argument part of
    the command line. With this option, user can specify which
    attribute will have None value.

    For example to set dns_nameservers of subnet to None, we use:
    quantum subnet-update subnetname --dns_nameservers action=clear

    Of course, client command developer can use known option like --no-gateway
    to do the same job.

    Change-Id: Ia8dffcfd1122259d545a61e71db25e459639d130