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.
My understanding is that anything we expect to be end-user facing goes in INSTALL_DIR/bin. If it is a utility that is used by the Chef server itself and not expected to be end-user facing, then it goes into INSTALL_DIR/embedded/bin. In looking at this, I see this applies to gather-logs and wait-for-rabbit. I'd expect these to be in embedded/bin, since they are used by other aspects of the backend server tooling, but changing that at this point would likely break other things and we should look at a fix outside of this PR.