Make xvfbwrapper import dependent on the selenium imports

It will not be required unless Selenium is also present.

Change-Id: I9da1c596ad68b68c3ce4e8cd215646209dbaebe7
Closes-Bug: #1336243
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>
Allow panel plugins to use ADD_EXCEPTIONS and UPDATE_HORIZON_CONFIG

This change allows panel plugins tell horizon which exceptions to treat as recoverable.
Previously only dashboard plugins were allowed to register exceptions.
Since there are now more settings that are common than specific to pluggable use case,
the doc was restructured to describe in general pluggable settings and then describe
the specific use cases.

Change-Id: I013ece33ae2b979e92be9c7ba50182025a1ecb91
Closes-bug: #1332688
Imported Translations from Transifex

Change-Id: Iee2faa595a349a7f41a701812365af6194765ebf
Merge "Make xvfbwrapper import dependent on the selenium imports"
Updated from global requirements

Change-Id: Iecce22d24ec2436d3cf0f7fd906cd5e16cf821cb
Merge "Allow panel plugins to use ADD_EXCEPTIONS and UPDATE_HORIZON_CONFIG"
Merge "Updated from global requirements"
Merge "Adding plugins panel for Sahara"