[networking-guide] add "enable neutron sriov-agent"
[networking-guide] add "enable neutron sriov-agent"

In [1], though "agent_required" is deprecated, in mitaka the agent is
always be required, it is still need to enable neutron sriov-agent

[1]: https://review.openstack.org/#/c/241730/

Change-Id: I7af77ee73bc00e9177ab648df2298395266f0a58
Closes-Bug: #1585132

openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals