Fix conditional test against useradd so it's compatible with Dash

The test to see which version of `useradd` is installed uses 'bashisms'
that fail on Ubuntu due to the fact that `/bin/sh` is symlinked to Dash,
causing the telegraf account to be created without the `--system` option
ever being passed.

This change amends the syntax so that it's POSIX-compatible and more
portable as a result.