This patch adds support for creating application credentials in
keystone[1]. Application credentials can be created by any user for
themselves. An application credential is created for the currently
selected project. A user may provide their own secret for the application
credential, or may allow keystone to generate a secret for them. After
the application credential is created, the secret is revealed once to
the user. At that point they may download a clouds.yaml or openrc file
that contains the application credential secret and will enable them to
use it to authenticate. The secret is not revealed again.

[1] https://docs.openstack.org/keystone/latest/user/application_credentials.html

bp application-credentials

Depends-On: https://review.openstack.org/557927
Depends-On: https://review.openstack.org/557932
Change-Id: Ida2e836cf81d2b96e0b66afed29a900c312223a4