Fail to add subnet to router if this subnet already add to this router

[tinytmy]

When we already add the subnet to router, if we click 'Add Interface' again,
we can see the subnet also can select, but if we real to add the subnet, it
report a error. We can hidden the subnet if it already add to the router.

Error: Failed to add_interface: Bad router request: Router already has a port
on subnet d5394fc5-1295-43ed-9336-0362ced9b8d6

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/154766

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/154766
Committed: https://git.openstack.org/cgit/openstack/horizon/commit/?id=ca596187a8cb9009dc1a6c8dffada97f651861df
Submitter: Jenkins
Branch: master

commit ca596187a8cb9009dc1a6c8dffada97f651861df
Author: tinytmy <email address hidden>
Date: Mon Feb 9 21:37:22 2015 +0800

    filter the subnets which already in the router

    When we already add the subnet to router,
    if we click 'Add Interface' again, we can see
    the subnet also can select, but if we real to
    add the subnet, it report a error. We can hidden
    the subnet if it already add to the router.

    Closes-bug:#1417892

    Change-Id: I485608d267b9b235c1e83a990e330dd1ca98f17e