Move deployment SSH key to stable directory

Summary:
/var/run is pruned at startup, so we need to put this key elsewhere.
This is the issue described in T1350.

Side-edit: document where to put the new public key

Test Plan: Deployed on Ysul

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1405