Extending Horizon's plug-in mechanism to scss

Summary:
Currently in Horizon's application part - openstack_dashboard, scss files are integrated to the app using dashboard.scss as the integration point and scss files for each enabled dashboard are imported into it by using hard-coded approach. This approach has some issues:

* style for disabled dashboard will still be included in the app, unless changing the source code
  during deployment.
* developer has to manually maintain the integration list or tree

Motivation:
We want to solve the issues listed above.

Description:
Extends Horizon's plug-in mechanism to scss, so to solve the above issue. This extended plug-in mechanism is based on a simple naming convention, just like it does with JavaScript files, HTML files
and Python files. The naming convention is:

* For each dashboard, `project` for example, there should be a directory
  defined for it: openstack_dashoboard/static/dashboard/project/.
* And there should be a `project.scss` defined for it

UX:
No impact.

Outside Dependencies:
N/A

Requirements Update Required:

Doc Impact: