feat(users): Add support for creating/modifying users

Documentation/cloud-config.md

@@ -13,6 +13,40 @@ Provided public SSH keys will be authorized for the `core` user.
 The keys will be named "coreos-cloudinit" by default.
 Override this with the `--ssh-key-name` flag when calling `coreos-cloudinit`.
 
+#### users
+
+Add or modify users with the `users` directive by providing a list of user objects, each consisting of the following fields.
+Each field is optional and of type string unless otherwise noted.
+All but the `passwd` and `ssh-authorized-keys` fields will be ignored if the user already exists.
+
+- **name**: Required. Login name of user
+- **gecos**: GECOS comment of user
+- **passwd**: Hash of the password to use for this user
+- **homedir**: User's home directory. Defaults to /home/<name>
+- **no-create-home**: Boolean. Skip home directory createion.
+- **primary-group**: Default group for the user. Defaults to a new group created named after the user.
+- **groups**: Add user to these additional groups
+- **no-user-group**: Boolean. Skip default group creation.
+- **ssh-authorized-keys**: List of public SSH keys to authorize for this user
+- **system**: Create the user as a system user. No home directory will be created.
+- **no-log-init**: Boolean. Skip initialization of lastlog and faillog databases.
+
+The following fields are not yet implemented:
+
+- **inactive**: Deactivate the user upon creation
+- **lock-passwd**: Boolean. Disable password login for user
+- **sudo**: Entry to add to /etc/sudoers for user. By default, no sudo access is authorized.
+- **selinux-user**: Corresponding SELinux user
+- **ssh-import-id**: Import SSH keys by ID from Launchpad.
+
+##### Generating a password hash
+
+You can generate a safe hash via:
+
+    mkpasswd --method=SHA-512 --rounds=4096
+
+Using a higher number of rounds will help create more secure passwords, but given enough time, password hashes can be reversed.
+
 ## Custom cloud-config Parameters
 
 ### coreos.etcd.discovery_url
@@ -80,3 +114,18 @@ coreos:
           [Install]
           WantedBy=local.target
 ```
+
+### Add a user
+
+```
+#cloud-config
+
+users:
+  - name: elroy
+	passwd: $6$5s2u6/jR$un0AvWnqilcgaNB3Mkxd5yYv6mTlWfOoCYHZmfi3LDKVltj.E8XNKEcwWm...
+	groups:
+	  - staff
+	  - docker
+	ssh-authorized-keys:
+	  - ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC0g+ZTxC7weoIJLUafOgrm+h...
+```