Also, thinking about this a bit more, we should clearly define what the default behavior is here. In an ideal world, there would be a global heka runtime folder (e.g. `/var/run/hekad/...`), and the seekjournals would default to automatically putting themselves in a specific place relative to the global heka root. The world isn't ideal, however, and heka doesn't yet support the notion of a "global heka runtime folder", but we should still probably pick a reasonable default for this.