Modify log action so JSON output is consistent

Without this, the output stream includes non-JSON text when running with `--json=true`.

When running with the text logger, this will now output like:

```
20:40:30.425383 log log-complete
20:40:30.425559 log log-complete Flynn bootstrapping complete. Install flynn-cli and paste the line below into a new terminal window:

flynn server-add -g localhost:2201 -p +dIcDZx3M232Bs0iOQsodJrx4Q/xmhangOP+kaZK2Sw= default https://localhost:8081 4128ebf9b7340e038ad2a83f183670a1
```

and for JSON:

```
{"id":"log-complete","action":"log","state":"start","ts":"2014-06-22T20:39:51.920202207Z"}
{"id":"log-complete","action":"log","data":{"message":"Flynn bootstrapping complete. Install flynn-cli and paste the line below into a new terminal window:\n\nfly
nn server-add -g localhost:2201 -p SHSmz8E8+zr1clBTlj39nxGzEAKuR4iYcoFtfqPeDjQ= default https://localhost:8081 fc0e2016d63d878aea31d0a5d6def7f1"},"state":"done","
ts":"2014-06-22T20:39:51.922019808Z"}
```