Add support for creating, reading, and deleting application credentials.
Application credentials do not support updating.
Keystoneclient does not handle authentication with application
credentials. This is done in keystoneauth. Additional work will be
needed in python-openstackclient to support both CRUD and auth for
application credentials.
bp application credentials
Change-Id: I21214238deac2c45f2f2d666287c2ae106955ab1