Adds an optional external_ip parameter to the prom join script API,
and to the Genesis and KubernetesNode schema.
This is used to populate the host's IP address in its /etc/hosts
file if present, according to normal hosts conventions.
If the value is not passed to prom-join or is absent from a
Genesis or KubernetesNode document, then the hosts file defaults
to the current loopback IP for the hostname (business as usual).
Change-Id: I58dc219923b18aaf9c83453b896ce509664d8766
c18e3a8d9d