Adding plugins panel for Sahara

The plugins panel is used to show the various
hadoop flavors/versions that are supported by the
underlying Sahara API.
* also added basic panel tests
This code was originally from:
https://github.com/openstack/sahara-dashboard

Change-Id: Ib0c95334a5122529fddbfd9c0cd0d0c06e0c8ad3
Partial-Implements: blueprint merge-sahara-dashboard
Co-Authored-By: Nikita Konovalov <nkonovalov@mirantis.com>
Co-Authored-By: Dmitry Mescheryakov <dmescheryakov@mirantis.com>
Merge "Adding plugins panel for Sahara"