host: Update configuration file format

Configuration files are now JSON describing the arguments and
environment variables to add when running the daemon.

Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>
host: Fix panic converting error in log

Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>
host: Add expose_env field to service manifest

Environment variables listed in this field are copied from the daemon
environment into the job.

Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>
host: Configure etcd clustering via environment variables directly

Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>
pkg: Add etcdcluster package

Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>
host/cli: Add init command for configuring the cluster

Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>
host: Default to 1-node etcd configuration if none is specified

Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>
host: Lookup default external IP

Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>