instead of 'newgrp' and a pipe, use 'sg'

sg and newgrp basically do the same thing, but sg takes a command to
execute rather than just executing sh. This just seems cleaner.

stack.sh

@@ -592,9 +592,8 @@ fi
 # have to do a little more than that in our script.  Since we add the group
 # ``libvirtd`` to our user in this script, when nova-compute is run it is
 # within the context of our original shell (so our groups won't be updated).
-# We can send the command nova-compute to the ``newgrp`` command to execute
-# in a specific context.
-screen_it n-cpu "cd $NOVA_DIR && echo $NOVA_DIR/bin/nova-compute | newgrp libvirtd"
+# Use 'sg' to execute nova-compute as a member of the libvirtd group.
+screen_it n-cpu "cd $NOVA_DIR && sg libvirtd $NOVA_DIR/bin/nova-compute"
 screen_it n-net "cd $NOVA_DIR && $NOVA_DIR/bin/nova-network"
 screen_it n-sch "cd $NOVA_DIR && $NOVA_DIR/bin/nova-scheduler"
 screen_it n-vnc "cd $NOVNC_DIR && ./utils/nova-wsproxy.py 6080 --web . --flagfile=../nova/bin/nova.conf"