try to fix LOGSTASH-2225

Change logic from using "chroot --userspec" (which doesn't work
with RHEL5) to using "sudo". Side effects:
1. It's no longer possible to explictely set the group that LS
   will use (LS_GROUP variable removed).
2. Related to #1, supplemental groups for logstash will now work.
3. Due to the way that sudo behaves, I had to shift logfile
   creation to the non-privileged part, so on upgrades, if one
   doesn't explicitely chown existing logfiles, it will break.

Regarding #3: We could probably chown the logfiles to $LS_USER from
within the init script...