Configurable domain

Create custom domain names with container environment variables that override skydns mappings for service, version, environment and ttl.

docker run -e DNS_ENVIRONMENT=space -e DNS_SERVICE=name -e DNS_VERSION=custom -e DNS_TTL=120 foo/bar
->
custom.name.space.skydns.local

This includes a small change to heartbeat and updateService to use the container specific ttl.