Allow timeouts to be specified on Drydock SSH connections

Summary: This allows timeouts to be specified on SSH connections that Drydock makes.  Used in the EC2 allocator to poll for the SSH server starting.

Test Plan: Used in EC2 allocator diff.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10225