combine keystores / adding certificates improvements

## Context
It should be easy to add your own certificates or CA's to Graylog.

Currently our Documentation recommend to copy or create a keystore and use that exclusive via startup parameters.

http://docs.graylog.org/en/2.4/pages/configuration/https.html#adding-a-self-signed-certificate-to-the-jvm-trust-store

While it would be more failsafe to add the own certificates to the system truststore it is not as easy as having the own exclusive. So document that is essential important.

But Graylogs security would benefit if it is more easy to add CA or certificates, via own truststore that is combined with the system provided one. To easy that even more up make this possible via the UI.

The benefit could also be that Graylog is using a internal CA for all self signed certificates and so the users would benefit because Graylog and all ingestors have a path of trust even without having the user his own CA.

## Your Environment
* Graylog Version: 2.4