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 coreos_install_device host parameter:
```
coreos_install_device: /dev/vda
```

Optionally you can provision ssh keys using the ssh_rsa host parameter:
```
ssh_rsa: ssh-rsa AAAAB...== root@somehost.local
```

In order to provision a discovery token for etcd you can set the discovery_url host parameter:
```
discovery_url: https://discovery.etcd.io/059fb72d2e690b847...
```





Maybe something like ```etcd_discovery_url``` is better, as the term "discovery" in Foreman is mostly associated with foreman_discovery.