Add support for creating, retrieving, and deleting application
credentials. Application credentials do not support updates.
In order to provide a positive user experience for the `--role` option,
this patch also includes an improvement to the
`identity.common._get_token_resource()` function that allows it to
introspect the roles list within a token. This way there is no need to
make a request to keystone to retrieve a role object, which would fail
most of the time anyway due to keystone's default policy prohibiting
unprivileged users from retrieving roles.
bp application-credentials
Change-Id: I29e03b72acd931305cbdac5a9ff666854d05c6d7
