horizon test helper prevents mox free horizon plugin

[Akihiro Motoki]

Currently horizon test helpers (horizon/test/helpers.py and openstack_dashboard/test/helpers.py) always imports mox3.
These test helpers are consumed by horizon plugins.
Even when horizon plugins are mox free, these test helpers requires mox3 and plugins cannot drop mox3 from test-requirements.txt. mox should be optional in horizon test helpers.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/549842

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/549842
Committed: https://git.openstack.org/cgit/openstack/horizon/commit/?id=fb9ef26a7b26f42c350b7a15e988ce58ce1da943
Submitter: Zuul
Branch: master

commit fb9ef26a7b26f42c350b7a15e988ce58ce1da943
Author: Akihiro Motoki <email address hidden>
Date: Tue Mar 6 03:44:31 2018 +0900

    Allow mox-free horizon plugins to consume horizon test helper

    horizon test helpers now depend on mox3.
    This prevents horizon plugins from consuming the test helpers
    even if a plugin itself is mox free.
    This commit makes mox import optional.

    Change-Id: I631518d8f4cd9641920f68cd1e405298ddb7965a
    Closes-Bug: #1753555

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/horizon 14.0.0.0b1 development milestone.