Support KnownHostsCommand for known hosts completion

[sebastianw]

Describe the feature/solution

Currrently the known hosts completion only supports known hosts file. When using KnownHostsCommand instead the completion has no hosts.

Maybe the completion could execute the command instead and get the information from there.

Maintenance (please complete the following information)

• [ ] This is a request for new completion
• [ ] Link to upstream project query about shipping the completion:

[akinomyoga]

UserKnownHostsFile and GlobalKnownHostsFile also seem to be the ssh_config options that affect listing hostnames.