Add support for various config options

- `advertise_addr`
- `bootstrap_expect`
- `encrypt`
- `watches`

Also, more control over `start_join`: don't assume both clients and servers want to join at start if they're not bootstrapping.  (Consul devs recommend against servers joining at start since it doesn't play well with respawn.)  Join at start is now only enabled if `consul_join_at_start` is defined and truthy.