Add exclusive key to distro .info file

Problem/Motivation

When installed in its fully packaged form, Open Outreach's installer includes the screen to select an install profile, with standard, minimal, and Open Outreach as options. This screen is superfluous, since presumably no one would download the packaged version in order to install vanilla Drupal core.

Proposed resolution

Specify the exclusive key in openoutreach.info. See https://api.drupal.org/api/drupal/includes!install.inc/function/install_....

Remaining tasks

User interface changes

API changes