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
Output from "ping" is distracting and irrelevant

... especially on QEMU, where we can't ping out, so it just prints to the console endlessly
Merge pull request #471 from tianon/init-issues

Fix several issues with our init script
Merge pull request #472 from tianon/ping-output

Output from "ping" is distracting and irrelevant
Speed up ssh access by turning off DNS checking
Merge pull request #467 from SvenDowideit/usedns-no

Speed up ssh access by turning off DNS checking