Allow changing any horizon settings from the plugin config files

Sometimes we want our pluggable dashboard to change more than just
the selected handful of options in the HORIZON_CONFIG. This patch
lets the configuration files for the plugins add and modify any
horizon settings.

Change-Id: I25aec577733631d1e2057bde0b12f1e50c56b05b
Closes-bug: #1302387
Merge "Allow changing any horizon settings from the plugin config files"