Allow custom nsesa-server.properties path

[joelpurra]

By setting the environment variable NSESA_SERVER_PROPERTIES or java
parameter -DNSESA_SERVER_PROPERTIES to a path such as
/any/path/nsesa-server.properties properties can be loaded from
anywhere in the file system. This is useful for production systems.

Loading from ${user.home}/nsesa-server.properties is still supported.

---

Edit: hard-coded file:// which means less typing, less flexibility and no null pointer exceptions when no value was set.