Fix issue with $MODE_PROD in server setup script

In shell, [[ false ]] returns 0 (i.e. evaluates to true). In
particular, [[ $MODE_PROD ]] always return true.

Instead of this, use an empty string for $MODE_PROD to indicate
non-prod mode.
Merge pull request #925 from learning-unlimited/server-setup-mode-prod

Fix issue with $MODE_PROD in server setup script