Put HostManager._instance_info on a diet

This is a specless blueprint to cleanup some technical debt between the compute manager and host manager in the scheduler.

The default scheduler driver is the filter scheduler, and the affinity/anti-affinity filters/weighers are enabled/loaded by default.

Those rely on the _instance_info cache in the HostManager which is either populated by the ComputeManager sending updates (configured via the CONF.filter_scheduler.track_instance_changes option) or by pulling that information directly from the database if the compute doesn't send it.

The only in-tree filters/weighers that use the _instance_info cache are the affinity ones.

There are a few issues here:

1. By default, the instance info stuff is always sent from compute to scheduler regardless of whether or not the filter scheduler is being used. If I'm using the caching scheduler, then this is just unnecessary RPC traffic.

2. The full instance objects are being sent over RPC but the only thing that the affinity filter/weigher needs is the uuid, because that's what InstanceGroup.members are, a list of instance uuids.

3. There is a _sync_scheduler_instance_info periodic task that runs on all computes by default every 2 minutes. That pulls all non-deleted instances from the database for that host (including unnecessary joins on the info_cache and security_groups tables) and sends the full list of instance objects over rpc to the scheduler (remember, only the uuids are used).

4. As in #1 on the compute, the HostManager in the scheduler by default tracks instance changes, even if not using the FilterScheduler. If mis-configured, and the computes aren't sending the full instance lists on startup, the scheduler will pull them from the database.

This is all obviously problematic and wasteful if (1) you're not even using the filter scheduler or using the affinity filter/weigher, and (2) even if you are, you don't need the full instance object and all of it's sub-objects sent over RPC every 2 minutes from all computes in the deployment. At any reasonably scaled deployment this is a lot of overhead.

In this blueprint we'll tackle the main issues by simply sending uuids insteand of full instance objects, and not actually do this work if not configured for the filter scheduler.

Note that any out of tree filters/weighers relying on the HostManager._instance_info cache of full instance objects to get things from the instances besides uuids would be impacted by this. However, if they are out of tree, they pay the price for changes to Nova out of their control. This isn't an API contract. Having said that, they will still have the instance uuids so if they need they can convert them to full objects from their filter if needed so they won't be totally broken.