horizon_workflow.scss could be more scss-like

horizon_workflow.scss is currently pretty much css.  There's some
semi-sophisticated css in there (the arrows) and it would be improved
with some variable usage and more nesting.

Based on my previous less-ify here:
https://review.openstack.org/#/c/82209/

NOTE: I've verified that the resulting .css file is exactly the same as
before.

NOTE 2: Per Maxime's suggestion, I've moved the variable declarations
into the _variables.scss file, and changed the format of a few of them.
The resulting .css file is now slightly different but only because
"white" is different from "#ffffff"

Change-Id: Ie31de779285c5f417c9cd7c8edc7ff5e7b527f5e
Closes-Bug: 1295847
Merge "horizon_workflow.scss could be more scss-like"