Create Subnet button respect quotas

[Bradley Jones]

To recreate:
                project has no available quota for subnets,
                click the network name,
                the create subnet button should be disabled

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/121935

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/121935
Committed: https://git.openstack.org/cgit/openstack/horizon/commit/?id=746af6eeb00ff6d0571de77362317ad7dab4d055
Submitter: Jenkins
Branch: master

commit 746af6eeb00ff6d0571de77362317ad7dab4d055
Author: Bradley Jones <email address hidden>
Date: Tue Sep 16 00:10:53 2014 +0100

    Create Subnet button should respect quotas

    The create subnet button (visible for each network in the networks list and on
    the detail page when looking at an individual network) should be disabled if
    the quota for subnets is at capacity.

    Tests have been added to check that when the quota is exceeded the buttons are
    disabled.

    Change-Id: I7bba89c2d89a39220900480f0091bdc6215505fe
    Closes-bug: 1369779