Be able to ignore a specific host

[shlomi-noach]

It should be in freno's power to skip/ignore metrics from a given host. As example, freno would have a /skip-host/<hostname> API endpoint. This would override any configuration-based listing of servers as well as dynamic HAProxy-based listing.

The operation would be undone with, say /recover-host/<hostname> or similar.

Also, freno should export the list of skipped/ignore hosts, e.g. via /skipped-hosts