[DOCS] Adding missing kernel modules for VPNaaS

Without adding the three kernel modules missing
in the network hosts, ipsec will fail because
he needs them in order to work.

With this modules it's possible to have VPNaaS
agent be working.

Change-Id: I0c0a7b46f6f486e8e18f741dd9c6a1c691e937af

doc/source/install-guide/configure-network-services.rst

@@ -135,7 +135,7 @@ The following procedure describes how to modify the
         - router
         - metering
 
-#. ````neutron_plugin_base`` is as follows:
+#. ``neutron_plugin_base`` is as follows:
 
    .. code-block:: yaml
 
@@ -144,6 +144,25 @@ The following procedure describes how to modify the
          - metering
          - vpnaas
 
+#. Override the default list of specific kernel modules
+   in order to include the necessary modules to run ipsec:
+
+   .. code-block:: yaml
+
+      openstack_host_specific_kernel_modules:
+         - { name: "ebtables", pattern: "CONFIG_BRIDGE_NF_EBTABLES=", group: "network_hosts" }
+         - { name: "af_key", pattern: "CONFIG_NET_KEY=", group: "network_hosts" }
+         - { name: "ah4", pattern: "CONFIG_INET_AH=", group: "network_hosts" }
+         - { name: "ipcomp", pattern: "CONFIG_INET_IPCOMP=", group: "network_hosts" }
+
+#. Execute the openstack hosts setup in order to load the kernel modules at boot
+   and runtime in the network hosts
+
+   .. code-block:: shell-session
+
+      # openstack-ansible openstack-hosts-setup.yml --limit network_hosts\
+      --tags "openstack-hosts-setup,openstack-host-specific-kernel-modules"
+
 #. Execute the neutron install playbook in order to update the configuration:
 
    .. code-block:: shell-session