scss variables overriding mechanism

[Jiri Tomasek]

The horizon.scss now @imports _variables after importing bootstrap, which prevents us to override bootstrap variables there. (see how scss variables overriding work http://robots.thoughtbot.com/sass-default)

The fix should move _variables import before bootstrap import in horizon.scss and move all variables defined at the beginning of horizon.scss into _variables.scss.

Also, variables in _variables.scss shoudl be organized into 2 groups for clarity:
1. bootstrap overrides - variables that override bootstrap ones
2. custom horizon variables - variables used by horizon that are not defined by bootstrap

IMHO, all variables in _variables.scss should be marked as !default, so they can be overriden in horizon plugins to maintain Horizon's customization ability.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/133667

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/133667
Committed: https://git.openstack.org/cgit/openstack/horizon/commit/?id=75e01d70a8021f08deeddcaa0c61ae33993c89f1
Submitter: Jenkins
Branch: master

commit 75e01d70a8021f08deeddcaa0c61ae33993c89f1
Author: Jiri Tomasek <email address hidden>
Date: Mon Nov 10 11:08:15 2014 +0100

    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