Plug-in architect should allow for spec files to be added

[Shaoquan Chen]

Currently the plug-in architecture only allows for JavaScript source code files added form apps/panels, we need to extend the architecture to allow for spec(unit test) files to be added to Jasmine spec runner as well.

Besides, the JS files order is not respected by the current architecture, but the order is significant, this need to be fixed too.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/153927
Committed: https://git.openstack.org/cgit/openstack/horizon/commit/?id=49af820365f4f7bae947d6388a09f86b3d7c5e41
Submitter: Jenkins
Branch: master

commit 49af820365f4f7bae947d6388a09f86b3d7c5e41
Author: Shaoquan Chen <email address hidden>
Date: Sun Feb 8 22:00:18 2015 -0800

    Extending Horizon's plug-in architecture to Jasmine

    Horizon has a well-defined plug-in architecture, but the
    architecture doesn't cover Horizon's Jasmine spec runner.
    This patch fills the gap.

    This patch also
    - defines a new field `js_spec_file` in HORIZON_CONFIG in settings.
    - fix a bug openstack_dashboard/utils/settings.py where using python set
      to collect js files, which will cause the order of the js file lost.
      Change is made to use list to respect the js files order.
    - update doc/source/topics/settings.rst to include the newly added field.

    Change-Id: Idc122e8b1f917cdd344aa8f4b48fd80a6bdddb70
    Close-Bug: 1421778