ctdb-tools: Avoid filtering stderr when onnode not running in parallel

ctdb-tools: Avoid filtering stderr when onnode not running in parallel

stderr_filter() only does anything useful when running in
parallel (i.e. with the -p option).  So, simplify the non-parallel
case by not using stderr_filter().

As a side-effect, this fixes an issue introduced in commit
85a4375788d8ef8345ec390807f18299abdadb20 where local daemon tests
would hang when trying to start daemons with VALGRIND set (to a
valgrind command that does not use --log-file).  This is because
valgrind would keep stderr open for its output so the pipeline
involving stderr_filter() would never complete.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>