Plugin-based panel configuration

This is an extension to the plugin-based dashboard configuration.
It adds support processing panel configuration configuration file
in the openstack_dashboard/enabled/ directory.

Panels can be added, removed to/from the panel group of a dashboard.
It also provide the ability to update the default panel of the
dashboard.

Change-Id: I2d7adfb8045c244ec063a6741e3b9fe21c188525
Implements: blueprint plugin-panel-config
Merge "Plugin-based panel configuration"