sh: Make use of bashisms in bash scripts

Both [[ ]] and (( )) bashisms are better than the alternative
in POSIX sh, since they are builtin and don't suffer from quoting
or number-of-args issues.

Signed-off-by: Manuel Mendez <mmendez@equinix.com>

deploy/vagrant/scripts/tinkerbell.sh

@@ -34,7 +34,7 @@ setup_nat() (
 main() (
 	export DEBIAN_FRONTEND=noninteractive
 
-	if [ ! -f ./.env ]; then
+	if ! [[ -f ./.env ]]; then
 		./generate-env.sh eth1 >.env
 	fi