feat(etcd): Write etcd systemd snippet

Documentation/cloud-config.md

@@ -74,12 +74,38 @@ Provide a list of objects with the following attributes:
 
 ## Custom cloud-config Parameters
 
-### coreos.etcd.discovery_url
+### coreos.etcd
 
-The value of `coreos.etcd.discovery_url` will be used to discover the instance's etcd peers using the [etcd discovery protocol][disco-proto]. Usage of the [public discovery service][disco-service] is encouraged.
+The `coreos.etcd.*` options are translated to a partial systemd unit acting as an etcd configuration file.
+`coreos-cloudinit` will also replace the strings `$private_ipv4` and `$public_ipv4` with the values generated by CoreOS based on a given provider.
 
-[disco-proto]: https://github.com/coreos/etcd/blob/master/Documentation/discovery-protocol.md
-[disco-service]: http://discovery.etcd.io
+For example, the following cloud-config document...
+
+```
+#cloud-config
+
+coreos:
+    etcd:
+        name: node001
+        discovery-url: https://discovery.etcd.io/3445fa65423d8b04df07f59fb40218f8
+        bind-addr: $public_ipv4:4001
+        peer-bind-addr: $private_ipv4:7001
+```
+
+...will generate a systemd snippet like this:
+
+```
+[Service]
+Environment="ETCD_NAME=node001""
+Environment="ETCD_DISCOVERY_URL=https://discovery.etcd.io/3445fa65423d8b04df07f59fb40218f8"
+Environment="ETCD_BIND_ADDR=203.0.113.29:4001"
+Environment="ETCD_PEER_BIND_ADDR=192.0.2.13:7001"
+```
+
+For more information about the available configuration options, see the [etcd documentation][etcd-config].
+Note that hyphens in the coreos.etcd.* keys are mapped to underscores.
+
+[etcd-config]: https://github.com/coreos/etcd/blob/master/Documentation/configuration.md
 
 ### coreos.units