Add systemd override to auto restart mongod
This ensures mongod is always auto restarted by systemd if it crashes or exits for some
reason. The 5 second wait is to reduce issues if mongod crashes quickly after startup.
(cherry picked from commit 16ec1a6f503e0bfcfee96b6b8a87914cdae45996) This is the master port from https://github.com/edx-olive/configuration/pull/20
see https://github.com/edx-olive/configuration/pull/20 (basically, run against a mongod node with the `install:configuration` tag. Run again to check idempotency.
[ ] @itsjeyd
[ ] edX reviewers TBD