Allowing read access to config via REST

Summary:
Support angular work by giving it read-only access to configs in settings via REST.

Motivation:
We have a lot of new angular widgets and panels coming, and some of them will need access to configs stored in settings. Right now, the default way to get them is through _conf.html or _script.html. We should really fetch them through the new REST layer instead. This ensures we are following the same pattern already set up for angular panels and services.

Example of getting config through _conf.html:
https://github.com/openstack/horizon/blob/master/horizon/templates/horizon/_conf.html#L47

Description:

Create a REST layer similar to a service (like keystone or nova) that would provide configurations. Configurations can be broken down into two categories:

1. Configuration for users (things like modal behavior, last page visited, language).
User configs are fetch once per user and stored client-side.
Future changes to this config will remain client-side.

2. Configuration for admins (things like help url, home url, disable password reveal).
Admin configs are fetch on page load.
As we move toward a single-page-app, this would be one fetch per session.
This is a read-only config and will be limited in scope (only selected configs will be expose).

UX:
N/A

Outside Dependencies:
N/A

Requirements Update Required:
N/A

Doc Impact:
N/A