[oslo][keystone][nova] Spec for moving policy format default to YAML

One of the outcomes of the Oslo PTG session on Monday was that we need to make YAML the official default for olso.policy instead of just the unofficial default as it has been since policy-in-code happened. The reason this hasn't happened before now is that it is complex and fraught with security concerns, but the RBAC work going on now has made it clear that we need do it anyway. To that end, I've written a spec[0] that I believe captures the plan we outlined in the PTG session. If this is relevant to your interests, please take a look and leave feedback. Thanks. -Ben 0: https://review.opendev.org/733650