[keyserver] Introduce concept of auxiliary node for running certain cronjobs

Summary:
We'll use the auxiliary node for running the the Comm team dev scripts (RUN_COMM_TEAM_DEV_SCRIPTS) and for database backup.

Depends on D13134

Test Plan: Ran this on my personal server and confirmed that it worked

Reviewers: will

Reviewed By: will

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13135