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.
Merge pull request #7 from waysact/more-config-vars

Add support for various config options