Use fixed device identifier instead of node identifier

[jensrossbach]

Please add the possibility to specify a fixed identifier for device nodes instead of taking the node identifiers. The problem is that node identifiers change when restoring flows from a backup or when moving nodes from one flow to another. Then as a consequence, the devices need to be discovered again (and possibly deleted beforehand in the Alexa app).

A possible solution could be to add a property to the device nodes where users can specify their own identifier and use the node identifier just as a default value for this property.