cloudinit: retrieve IPv4 addresses from metadata

This uses the new MetadataService implementation to retrieve values for
$private_ipv4 and $public_ipv4 substitutions, instead of using
environment variables.
initialize/env: fall back to COREOS_*_IPV4 env variables
initialize/env: add notes about tests
Merge pull request #158 from jonboulle/nettttt

cloudinit: retrieve IPv4 addresses from metadata