Disable known hosts when SSH'ing through Drydock

Summary: This prevents SSH from saving the host key into known_hosts; StrictHostKeyChecking only prevents it from prompting for unknown hosts, but it will still verify hosts against what it has previously saved.

Test Plan: N/A

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10252