Configure ssl certificate and key path via environment variables

[iesen]

It would be perfect if we set these properties in neo4j-server.properties using environment variables.

[benbc]

@iesen That's a great idea, thanks. We'll try to get it in soon.

[srbaker]

We have added the ability to provide SSL certificate and key in 01d3407 .

If you provide the key and certificate, and name them appropriately, they will get used automatically.