doc: Describe running a command as a separate group

Some commands must be run as a separate group to work. Users can use the
'sg' tool to do this.

This may be assumed knowledge for many users but it's helpful to note in
this, the definitive resource for DevStack's systemd integration.

Change-Id: I271c1d21b44fa972c152780c1caa01c21c265159
Signed-off-by: Stephen Finucane <sfinucan@redhat.com>

Author: stephenfin

doc/source/systemd.rst

@@ -152,6 +152,19 @@ Invoke the command manually::
 
   /usr/local/bin/nova-scheduler --config-file /etc/nova/nova.conf
 
+Some executables, such as :program:`nova-compute`, will need to be executed
+with a particular group. This will be shown in the systemd unit file::
+
+  sudo systemctl cat devstack@n-cpu.service | grep Group
+
+::
+
+  Group = libvirt
+
+Use the :program:`sg` tool to execute the command as this group::
+
+  sg libvirt -c '/usr/local/bin/nova-compute --config-file /etc/nova/nova-cpu.conf'
+
 Using remote-pdb
 ----------------