Use the 'US/Pacific' timezone

The PDT timezone doesn't exist, causing some weird stuff to happen when Puppet logs to syslog.  Use the US/Pacific timezone instead.

It would be nice to progammatically determine the timezone for this, but I know of no way to do this in a cross-platform manner.  Ideas I considered were:
1) read /etc/timezone (doesn't have to match current timezone, isn't used across all Linuxes)
2) somehow reverse-engineer a correct value of TZ from the contents of /etc/localtime.