Duplicate validation error for 'image_location'

[Liyingjun]

When enter an invalid url to 'image location' when create image, the validation error message occurred twice.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/256346

[Lin Hua Cheng (lin-hua-cheng)]

targetting to next milestone since this is a regression introduced this cycle

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/256346
Committed: https://git.openstack.org/cgit/openstack/horizon/commit/?id=bb66ec17b1ec4c70813a95236d0479c3de658c26
Submitter: Jenkins
Branch: master

commit bb66ec17b1ec4c70813a95236d0479c3de658c26
Author: lin-hua-cheng <email address hidden>
Date: Fri Dec 11 03:03:51 2015 -0800

    Fix duplicate Image URL validation message

    URLField already have a default_validator of URLValidator,
    defining another validator appends to the default_validator
    causing duplicate message.

    We should override the default_validator for restricting the
    url schema rather than appending another URLValidator.

    Change-Id: Icf09b2a70e63f610e34212ddc4d74f077a07b089
    Closes-bug: #1525100