Keystone role should use keystone-manage bootstrap

The Keystone team has recently added https://github.com/openstack/keystone/blob/d2070cdd18af4132c1ff2d651ee869c3156c3874/doc/source/configuringservices.rst#setting-up-credentials-with-keystone-manage-bootstrap which duplicates a fair bit of Ansible logic contained within the Keystone Ansible role.

Our role should be modified to consume this Keystone feature, rather than duplicate its function.

All other roles that were making use of the admin token should switch over to using the admin username and password credentials.