nifi.sh doesn't work as expected when /bin/sh isn't bash

NIFI-8123 introduced a --wait-for-init option to nifi.sh, making use of some Bash and Zsh-only builtins such as declare. Currently nifi.sh points to /bin/sh though and even when bash is available it can point to other shells such as ash and dash.

This creates some noise during a regular startup with `start` and the new wait functionality doesn't work with a `start --wait-for-init` with other shells. Startup still succeeds however.