Add check which audits for Allocations against resource providers which are invalid

[Drew Freiberger]

Due to various failures of VM deployments and migrations, we sometimes have stale resource allocations against resource providers in the placement/nova_api database which cause "ghost" contention when scheduling resources.

This charm should audit all allocations against each resource provider and ensure that the allocations' instance is "not deleted or shelved" and "exists on the hypervisor matching the resource provider".

[Drew Freiberger (afreiberger)]

One quick-win check for this could be ensuring allocation count for a given provider matches the number of vms/servers scheduled to the host:

$ os resource provider show $server1_resource_provider_uuid --allocations --format yaml | grep resources | wc -l
37
$ os server list --all --host $server1_fqdn -fvalue | wc -l
28