[New] Use HashiCorp Vault to Manage Secrets

[tylerjl]

Hey Linode team! Here's a new guide walking through deploying Vault. The steps here are a blend of HashiCorp's best practices and my own experiences running Vault in production, and I've walked through the guide a few times on different distributions with success so I think it's ready for review.

Note that I added a minor patch to the Certbot guide. I ran into an issue wherein nginx was not present on CentOS 7, so this fixes the issue going forward (I'm not sure why the certbot package isn't correctly marking nginx as a dependency, but this change should be safer in case there's any upstream changes in the future.)

/cc @Guaris for visibility

[sagesyr]

Thanks Tyler! We've created a new PR for this and will have it available soon.

#2463