Introduce a plugin architecture for Horizon

mrunge: IMHO, the linked bug regarding dashboards in horizon config can be fixed as well.

mrunge: currently, I see no reason to have "plugins" at all (or why to give some kind of python modules a different treamment as others). So I propose here:
- treat all python modules below openstack_dashboard/dashboards as "plugins". More specifically: copying a python module to this location, should add it automatically to INSTALLED_APPS, and should also add it (if appropriate) to HORIZON_CONFIG as well.

mordred: I believe we can do what you're wanting in a packaging friendly way. We use entrypoint loading for the rest of openstack for this sort of thing, and I believe a very simple entrypoint registry would take all of about 5 minutes to put together. Alternately, you could make openstack_dashboard/dashboards a namespace package and get a similar effect. You _don't_ want to do this by copying modules in, because of the complications you'll have around devtest and the __init__.py files (there's a reason namepsace packages and entry point exist)

mrunge: right. I didn't meant to duplicate code, to copy system packages, etc. It is just intended to easily enhance the dashboard, without requiring to change settings.py after all. The latter is inherited from Django framework.

Desgin session notes: https://etherpad.openstack.org/p/icehouse-summit-Plugin-architecture-for-Horizon

Gerrit topic: https://review.openstack.org/#q,topic:bp/plugin-architecture,n,z

Addressed by: https://review.openstack.org/56367
    Plugin-based dashboard configuration

Addressed by: https://review.openstack.org/76881
    Remove apps from settings

Addressed by: https://review.openstack.org/83977
    Fix documentation about adding a new dashboard