Update gather-logs and migration scripts to honor postsgresql['username']

Cherry-picked two commits from 11-1-stable branch for migrations and fixed two migratosr added since then (014 and 016) to use the configured postgres_unix_user.

Added a recipe to create a defaults shell at /etc/opscode/private-chef.sh for gather-logs and other scripts. Updated gather-logs to use that file, if it's there.

The opscode-pgsql string is still hardcoded at
  https://github.com/opscode/oc_authz_migrator/blob/master/scripts/opc-run.sh#L50 
so I'll patch that to use /etc/opscode/private-chef.sh also.
What's going on here? We're checking if dig is on the system and if it is, run these commands, otherwise it's okay to not run the commands?

which also has a -a option that returns a 0 or 1 and no other output if the command is found/not found. Might be useful here instead of piping to /dev/null. At least, man on my mac tells me which has this option, so I'm assuming other distros have it as well.