Forget instance: accept fuzzy/partial hostnames

[shlomi-noach]

Previously, forget required fqdn of the instance to forget. The reasoning was that we wanted to avoid resolving the name, which would lead us to forgetting a different name than the name submitted to be forgotten, e.g. if we wanted to forget an old DNS name.

This PR now allows fuzzy/partial names in forget. orchestrator will attempt to find a single-matching instance, and without running name resolving.

Thus, orchestrator-client -c forget -i my-host-123 will succeed if a hostname my-host-123456.example.com exists and is the only server to match the partial name.

If both my-host-123456.example.com and my-host-123abc.example.com exist, that's an ambiguity and the operation aborts.

cc @ggunson