run: do not block on greenlets after command exits

run: do not block on greenlets after command exits

The stdout/stderr greenlets will not necessarily exit when the command does if
child processes are stuck in an uninterruptible sleep. For example, the fsx.sh
workunit spawns fsx processes that may be left behind in the D state after
/bin/timeout kills fsx.sh. These are connected to the stdout/stderr pipes which
prevent the greenlets from exiting normally.

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>