Remove Keystone Dependency from Castellan

Problem Description
===================

Currently in Castellan in order to obtain access to Barbican via the Barbican
Key manager a `context` containing a valid Keystone `auth-token` must be
provided.

The Swift Keymaster under development[1] will access Castellan in order to
obtain Secrets from Barbican, but would like to have a Keystone service user
with access to all keys and in the future be able to have independent user
access. For a service user, a `context` containing a Keystone `username`
and `password` will be used. Castellan must support this.

Proposed Change
===============

The proposed change is to allow Castellan to be able to check the context for
the Barbican Key Manager and be able to determine what type of Keystone
authentication function to use.

There will be a new type of hierarchal context object which will be passed from
the user/service to Castellan. It will be used instead of oslo.context.

The hierarchal context will consist of a `Credential` object as the parent
class and the children will be:

1.) `TokenCredential`, for authenticating with a token.

2.) `PasswordCredential`, for authenticating with a username and password.

3.) `CertificateCredential`, for authenticating with a certificate.

The context is first checked to see what type of object it is, after that we
determine which Keystone auth-type to use.