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.
The rsync command on the next line has been installing into INSTALL_DIR/bin since the beginning of time. So the above mkdir is now consistent with the rsync. 

Which raises the question: do these scripts go in bin/ or embedded/bin. For consistency with the chef-client install, chef-specific binaries should go in bin/, while embedded/bin is where we put the requisite utilities, etc.