Expose back end independent abstract capabilities to users

Regular OpenStack users (human or automated agents) need to discover backend independent abstract capabilites carried by the volume types that they use to create volumes. To that end, we propose defining a set of ``user visible`` extra specs which would then be shown to regular users (members or readers of keystone projects) when they list volume types or extra specs, and when they show extra specs. Extra specs that control specific back end features, select particular back ends, or reveal back end specifics would still only be available to administrators.