Add more flexibility around the prefixes available

This allows you to specify a custom prefix, or a tmp prefix which is
chosen automatically.

test/shell/t5.sh

@@ -1,7 +1,7 @@
 #!/bin/bash -e
 
 # should take slightly more than 35s, but fail if we take 45s)
-timeout --kill-after=45s 40s ./mgmt run --file t5.yaml --converged-timeout=5 --no-watch --allow-tmp-prefix &
+timeout --kill-after=45s 40s ./mgmt run --file t5.yaml --converged-timeout=5 --no-watch --tmp-prefix &
 pid=$!
 wait $pid	# get exit status
 exit $?