[networking-guide] Add the "restart l3_agent" on compute nodes section
[networking-guide] Add the "restart l3_agent" on compute nodes section

Change-Id: I7863017c6013424ab7134fbee024cc0c6a21bf97
Closes-Bug: #1648106

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals