Add a command to add new hosts to an existing deployment

When adding new hosts to an existing deployment, it is common to use the --limit argument to apply changes only to that host. There are issues with this, in particular for the bootstrap-servers command which needs to add the new host to /etc/hosts of all hosts. Without an entry in /etc/hosts for a compute node (and no DNS for the name), migration of VMs to/from that host will fail.

We should add a new command that will update /etc/hosts on all hosts, when new hosts are added. Some containers (e.g. nova_libvirt) may need to be restarted or recreated to pick up the new hosts entries.