recon: only query each host once for disk usage
recon: only query each host once for disk usage

`swift-recon -d` asks each server:port pair about its disks, which
means if you have servers_per_port enabled it will multiply-count the
disks on any servers that have more than one port enabled.

We only want to query each host once, and don't care which of the
available ports we use, so filter the hosts set to ensure each host
only appears in it once.

Closes-Bug: 1947852
Change-Id: Idd517e97beab7813fc782b60cc01bcc20cd54c24

openstack/swift,openstack/swift,tipabu/swift,tipabu/swift,openstack/swift,tipabu/swift,openstack/swift,tipabu/swift