Plugin-based dashboard configuration

This is a proof of concept of how plugin-based configuration of
OpenStack dashboards could be realized. Additional plugins add
their configuration file in the opestack_dashboard/enabled/
directory (the exact location is up for discussion, we will
probably want a whole hierarchy of locations, with the possibility
of overriding settings).

An example file _50_tuskar.py is added to show how an additional
dashboard could plug into this system. If you have tuskar_ui installed,
this would add Tuskar to Horizon.

Change-Id: Iaa594deffbdb865116f6f62855c32c8633d2b208
Implements: blueprint plugin-architecture
Merge "Plugin-based dashboard configuration"