Hide "Warning: Permanently added ... to the list of known hosts." in Harbormaster

Summary: Ref T1049. This messages is always printed to standard error now that the known hosts file is set to /dev/null. This hides the warning so that we'll be able to parse stderr for Windows hosts (where Powershell decides to output XML...)

Test Plan: Tested locally and verified the warning no longer appears.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D10362