Allow specifying the server name via annotation

[Phoenix616]

This adds the ability to specify an annotation to be used to get the preferred name of a server instead of just a label as labels have a relatively short maximum length. (63 characters)

The annotation uses the same key as the label (kuvel.azisaba.net/preferred-server-name by default) and if the annotation is set it will override the label, otherwise the label will be used like before.

I also added the missing label-key-prefix config option to the readme.