Add new v3 resource to provide for Kerberos authentication

Kerberos is expensive to perform on every request. A special resource should be created so mod_auth_kerb can be configured to authenticate using Kerberos. Subsequent commands can use the existing authentication token (preferably stored in a python-keyring) until it expires. Keystone is already aware when it doesn't have a valid token and will automatically retrieve authentication via the /tokens resource. It is proposed that a /tokens/kerberos resource be created and mod_auth_kerb be configured to require Negotiate authentication on that. The response will be similar to that of a username/password authentication, either a 401 or an X-Auth-Token.