Add a centralized palette to Horizon

    With a centralized palette it will become easier to customize
Horizon design. Colors and file syntax are inspired by BootstrapV3
variables.less file

Change-Id: Iee0ed7cfea03fbba3385787646e55dd96a424327
Implements: blueprint centralized-color-palette
Co-Authored-By: miziel-r <robert.mizielski@cloudwatt.com>
Merge "Add a centralized palette to Horizon"