Add privsep-helper to neutron sudoers file

With the new oslo-privsep library, there is now a
privsep-helper command that is used to escalate
privledges.

This command needs to be runnable by the neutron user
via sudo without a password. The old rootwrap command
is still used as well, so for now we need to have
both.

Change-Id: I8e9743da3e51e71a113d958c22007cf54aa17fc4

templates/sudoers.j2

@@ -5,3 +5,4 @@ Defaults:{{ neutron_system_user_name }} secure_path="{{ neutron_bin }}:/usr/loca
 
 {{ neutron_system_user_name }} ALL = (root) NOPASSWD: {{ neutron_bin }}/{{ neutron_service_name }}-rootwrap
 {{ neutron_system_user_name }} ALL = (root) NOPASSWD: {{ neutron_bin }}/{{ neutron_service_name }}-rootwrap-daemon
+{{ neutron_system_user_name }} ALL = (root) NOPASSWD: {{ neutron_bin }}/privsep-helper