Currently horizon defines default values of settings in the logic
using getattr(settings, <setting name>, <default value>) and
it is not easy to handle the default values of available settings.
This commit starts the effort to define default settings explicitly.
This is a preparation for ini-based-configurations.
It covers settings in openstack_dashboard/api.
Part of blueprint ini-based-configuration
Change-Id: Id4c3287f0a572fd14ea93b54bcab8fabda39e583