cmd/cron: use secrets package for database password
cmd/cron: use secrets package for database password

Plus some additional cleanup and updating of instructions.

Fixes b/129370804

Change-Id: If48070db058b8e7156bb0f6ab1c51ce8031c3ad1
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/454607
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite