Fix several issues with our init script

- replace `"` with `'` as appropriate
- allow `DOCKER_DIR` to be configured via "/var/lib/boot2docker/profile"
- add `DOCKER_LOGFILE` variable to control where the daemon logs go, and switch them from overwrite to append

cc @tiborvass (who pointed me at this script with questions about logging and made me look into it a bit more again)
:+1: