Reorganize variables to enable overriding

This patch enables scss variables to be overriden. It also cleans up unnecessary variables
defined in _variables.scss because of previous wrong overriding.
More details on

Closes-Bug: 1390101
Change-Id: I5078fe46ea6a992a096e2e23118a20cb199b7942
Remove duplicated headings from horizon tables

Many panels in horizon only contain a single table, in these cases the
tables often have the same name as the panel, leading to duplicated
headings, e.g. Instances (panel heading) then Instances (table
heading). This patch adds an attribute the table to hide or show the
title, since the majority of panels have this duplication this flag
is set to hide the heading by default allowing a developer to re-enable
it for specific cases.

Tabbed panels also often lead to duplicated titles between the panel
heading and the tabs themselves, that is not fixed, however how this
patch reduces the duplication in the case of Volumes (panel name),
Volumes (tab name) and Volumes (table name), by hiding the table
heading.

Change-Id: I65efa53cc881417288e5c5b29bfa7d539e97c7dd
Closes-Bug: 1379044
Fix TemplateSyntaxError at hypervisors view

If somehow nova API service-list isn't available, listing
hypervisors at /admin/hypervisors/ ends with TemplateSyntaxError.

Expected result is that the page should load but user will
see empty list and error message.

This fix is based on idea found in other views:

openstack_dashboard/dashboards/admin/...
images/views.py
aggregate/views.py
falvors/views.pu

that on any exception while getting data (get_data), we just
return empty list and handle exception to display error message.

Closes-Bug: #1392316

Change-Id: Ia665111185abecab6a96e6c72a6088cf2fa2e02b
Merge "Remove duplicated headings from horizon tables"
Documentation error in base tables link_classes

In /horizon/tables/base.py there is a mistake in the
documentation here:
https://github.com/openstack/horizon/blob/master/horizon/tables/base.py#L197

The name of the attribute is "link_classes", not
"classes" like the example suggests. That could be confusing for developers.

Change-Id: I61c1807f61364a152a5a51fcec8e9158dfa7067c
Closes-Bug: #1392721
Merge "Fix TemplateSyntaxError at hypervisors view"
Merge "Reorganize variables to enable overriding"
Merge "Documentation error in base tables link_classes"
Merge branch 'master' of https://github.com/openstack/horizon