Use a different location for auto-generated self-signed certificates

This is an issue if using self-signed certs for inputs.
The files apparently end up in /tmp which can be a problem if some job cleans old files on some distributions.

```
2015-11-30T14:17:43.726+01:00 WARN [AbstractNioSelector] Failed to initialize an accepted socket. java.io.FileNotFoundException: /tmp/keyutil_0.0.0.0:null_4781007538124841383.crt (No such file or directory) at java.io.FileInputStream.open0(Native Method) at java.io.FileInputStream.open(FileInputStream.java:195) at java.io.FileInputStream.(FileInputStream.java:138) at org.graylog2.plugin.inputs.transports.util.KeyUtil.initKeyStore(KeyUtil.java:116) at org.graylog2.plugin.inputs.transports.AbstractTcpTransport$1.createSslEngine(AbstractTcpTransport.java:194) at org.graylog2.plugin.inputs.transports.AbstractTcpTransport$1.call(AbstractTcpTransport.java:175) at
org.graylog2.plugin.inputs.transports.AbstractTcpTransport$1.call(AbstractTcpTransport.java:171) at
org.graylog2.plugin.inputs.transports.NettyTransport$1.getPipeline(NettyTransport.java:116) at org.jboss.netty.channel.socket.nio.NioServerBoss.registerAcceptedChannel(NioServerBoss.java:134) at
org.jboss.netty.channel.socket.nio.NioServerBoss.process(NioServerBoss.java:104) at
org.jboss.netty.channel.socket.nio.AbstractNioSelector.run(AbstractNioSelector.java:337) at
org.jboss.netty.channel.socket.nio.NioServerBoss.run(NioServerBoss.java:42) at
org.jboss.netty.util.ThreadRenamingRunnable.run(ThreadRenamingRunnable.java:108) at
org.jboss.netty.util.internal.DeadLockProofWorker$1.run(DeadLockProofWorker.java:42) at
java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) at
java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) at
java.lang.Thread.run(Thread.java:745)
```