Added templates for CoreOS support - feature 7652

Community Templates for CoreOS in addition to https://github.com/theforeman/foreman/pull/1890 (Foreman CoreOS support).

By default the templates assume that coreOS should be installed to /dev/sda. You can overwrite the behaviour by setting the install-disk host parameter:
```
install-disk: /dev/vda
```

Optionally you can provision ssh keys using the ssh_authorized_keys host parameter:
```
ssh_authorized_keys: ssh-rsa AAAAB...== root@somehost.local
```

In order to provision a discovery token for etcd you can set the etcd_discovery_url host parameter:
```
etcd_discovery_url: https://discovery.etcd.io/059fb72d2e690b847...
```





Like this?
```
              ExecStart=/usr/bin/systemctl enable docker-tcp.socket
<% if @host.params['ssh_authorized_keys'] -%>
      ssh_authorized_keys:
  <% @host.params['ssh_authorized_keys'].split(',').map(&:strip).each do |ssh_key| -%>
      - "<%= ssh_key %>"
  <% end -%>
<% end -%>
```