neutron.conf needs lock_path set for router to operate

This change is adding required configuration in neutron.conf
to set the lock_path parameter, which was missing in
compute-install-ubuntu.rst

Change-Id: If090bdf060dfe21d11b1a5dfd010dc8167d9e45e
Closes-Bug: #1796976
(cherry picked from commit f4d438019e)

doc/source/install/compute-install-ubuntu.rst

@@ -78,6 +78,18 @@ authentication mechanism, message queue, and plug-in.
        ``[keystone_authtoken]`` section.
 
 
+* In the ``[oslo_concurrency]`` section, configure the lock path:
+
+  .. path /etc/neutron/neutron.conf
+  .. code-block:: ini
+
+     [oslo_concurrency]
+     # ...
+     lock_path = /var/lib/neutron/tmp
+
+  .. end
+
+
 
 Configure networking options
 ----------------------------