Support text-based private key credentials in DrydockSSHCommandInterface

Summary: This updates DrydockSSHCommandInterface to correctly hold open the private key credentials for the life of the interface so that remote commands will execute correctly with a text-based private key.

Test Plan: Created a text-based private key, created a resource based on it and leased against it.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4111

Differential Revision: https://secure.phabricator.com/D7700