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.
Comments here might be good. It takes some parsing to realize that the first -c applies to the su and the second -c is wrapped in the command to be run by su and is the -c for psql.