On CoreOS where there is no package management, perform zero-trip
loops instead of throwing an exception for iterating over a member
of an undefined variable.
A freshly-installed CoreOS system does not always have a hostname configured.
This causes problems for etcd and BGP mesh configuration for Calico.
Assign the Ansible inventory name as hostname as part of CoreOS bootstrap,
if the hostname is the default ("localhost").
Stig Telfer