Feature cloud config file

Extended cloud-config.yaml file with some common and sane settings.

Added support for private network.
If private network is available it is used for etcd peer address unless `CROSS_CLOUD=1` is set.

Basic settings for fleet:
`HOSTNAME=coreos01`
`METADATA="disk=ssd,provider=digital-ocean"`
and etcd:
`DISCOVERY=https://discovery.etcd.io/<token>`