Pass cluster.instance to ssh-exec if it is defined

Summary: Ref T7034. This is a second special case, like commit hooks, where we need some help from Phabricator to make instance identity knowable.

Test Plan: Connected to an instance and ran SSH commands.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7034

Differential Revision: https://secure.phabricator.com/D11539