Add attribute to set site branding for Horizon

Adds the SITE_BRANDING variable to local_settings.py,
controlled by node["horizon"]["site_branding"].

Defaults to "OpenStack Dashboard".

(cherry picked from commit 1f7c4d8a86ac699befeba8456e2f7629f900864d)
Merge pull request #155 from claco/add-branding-attribute

Merged automatically by jenkins after successful build test