Create cashew log directory from pre-start stanza in upstart script.

BUG=chromium-os:5253
TEST=Stop cashew (sudo /sbin/stop cashew). Delete log directory (sudo rm -rf /var/log/cashew). Start cashew (sudo /sbin/start cashew). Observe that log directory has been created. Restart cashew (sudo /sbin/restart cashew). Observe that existing log directory has not been disrupted.

Review URL: http://codereview.chromium.org/3036032