PTFM-12572 Added test that checks MOTD strings

Summary: Added a dx ssh unit test that uses pexpect to check for MOTD strings, then exits the SSH session and terminates the job.

Test Plan: Ran the test locally on Ubuntu 14.04 and OS X 10.9 hosts.

Reviewers: akislyuk

Reviewed By: akislyuk

Subscribers: klai

Differential Revision: https://phabricator.dnanexus.com/D40