node: config: make wakeonlan a property string

node: config: make wakeonlan a property string

Moves the wakeonlan property to be a property string, with current mac
address as default key. This allows to later add further optional
properties such as bind-interface and broadcast-address.

Adds the `get_wakeonlan_config` helper function to parse the string
when read from the node config.

Signed-off-by: Christian Ebner <c.ebner@proxmox.com>