Security: Infrastructure vulnerability scanning

Created by: andreeleuterio

Scanning our infrastructure for vulnerabilities is part of the larger goal of enabling private code on Sourcegraph Cloud. The goal is deploying a tool to scan our servers for vulnerabilities. Tennable Nessus is likely the way to go here.

Acceptance criteria:

• We have a list of servers to scan and process in place to keep it updated.
• We have deployed the tool of choice to scan our servers periodically and store the results.
• We have a process and documentation for analyzing results and taking action on finds.