Improper error message during create subnet

[Doug Fish]

Admin->System->Networks->[click a network name]->Create Subnet

Provide a name, click next. The message 'Specify "Network Address" or clear "Create Subnet" checkbox.' is displayed, but no Create Subnet checkbox is present.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/125304

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/125304
Committed: https://git.openstack.org/cgit/openstack/horizon/commit/?id=3243486ef43191699d54da6cff4d244801a58944
Submitter: Jenkins
Branch: master

commit 3243486ef43191699d54da6cff4d244801a58944
Author: Sam Betts <email address hidden>
Date: Wed Oct 1 10:46:52 2014 +0100

    Override Error Message in Create Subnet Workflow

    The Admin->Networks->Create Subnet and Project->Networks->Create Subnet
    both inherited their error message from the Project->Create Network
    workflow, this patch moves the error message into a instance variable
    that can be overridden by sub-classes, and overrides it in the Create
    Subnet workflow.

    Change-Id: I87e9b89b68560291e389a24be86f788167c5d149
    Closes-Bug: 1375783