Enhance current key situation

[jcgruenhage]

Right now you need to specify 4 keys in the vars, this needs to be documented properly. Apart from that, we could also just create random keys when there is no config file and fetch the keys from the config file if a file exists, meaning we don't need to specify them anymore.

[jcgruenhage]

go run misc/genkeys.go for encryption keys,
go run misc/genkeys.go -sig for signing keys,

[jcgruenhage]

I've written a small key generator, the binary is available under misc/genkeys in this repo, the source in contrib/ansible/genkeys.go in the upstream yggdrasil-go repo.