Scrubber refactor

Glance-scrubber get db data through glance-registry V1 Client for a long time. Now we meet some problem:
1. The glance-registry V1 Client doesn't support Keystone V3 auth which is now used widely by default. (There is no domain related action in scrubber.)
2. As we all know. Glance now uses API V2 and sqlalchemy db by default. Even when use registry for db query, Glance suggest users to use registry V2.
3. Glance-registry is going to deprecated through the bp "Deprecate Glance Registry"

So it seems there is no value to scrub images through glance-registry V1 now.

This bp will do some refactor for glance-scrubber. Such as call glance-api directly and let glance-api decide which kind of db query will be used. Or using glance-registry V2 is another way.

We need to have a discussion about the solution first.

(rosmaita): We discussed this at the PTG and decided that:
(1) this is a priority for Queens, as we cannot remove the v1 registry until after the scrubber no longer uses it
(2) we don't want the scrubber to use the registry at all, as we are planning to remove the registry entirely