Implement "Run Remote Command" build step implementation.

Summary: This adds a build step implementation for running a command on a remote machine over SSH. It supports merging in various variables about the build (such as the commit hash / revision ID, repository call sign, version control type and clone URI).

Test Plan: Configured a build plan to run /bin/true on localhost and the build passed. Configured a build plan to run /bin/false on localhost and the build failed.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D7519