Undocumented: Format for encrypted keyfiles for TLS inputs

It is not clear, what encondings/formats are accepted for encrypted keyfiles for TLS enabled inputs.
Provinding PEM (ASN1 DER + base64?), DER and pkcs#8 yielded variations of "Invalid key file provided".

The stack trace mentions a wiki page[0] from netty.io. Following the instructions there, the pkcs#8 was generated, but with a passphrase and without luck.

It would be good to know if there is a known working format, and if so, we should document it somewhere prominently.

[0] https://netty.io/wiki/sslcontextbuilder-and-private-key.html

* Graylog Version: 3.1