Authentication plugins mechanism

Keystone currently has the ability to authenticate users by using external
authentication [1,2], rather than keystone authentication. This blueprint
aims to implement a mechanism for pluggable additional authentication
mechanisms for the nova client, so that new authentication modules can be
used easily (http basic, X.509 authentication, etc.)

[1] https://blueprints.launchpad.net/keystone/+spec/pluggable-identity-authentication-handlers
[2] http://docs.openstack.org/developer/keystone/external-auth.html