Assigning to networking for clarification on what needs to be done in config. We also need to consider what we need to do during upgrades. Cluster Lifecycle team can help implement changes but we need to be told exactly what to do.

no clusterNetworks config information found in v3.5.5.31.34 either, look like this is not a bug.

Log from oc v3.5.5.31.34
[root@ip-172-18-0-99 ~]# oc version
oc v3.5.5.31.34
kubernetes v1.5.2+43a9be4
features: Basic-Auth GSSAPI Kerberos SPNEGO

Server https://ip-172-18-0-99.ec2.internal:8443
openshift v3.5.5.31.34
kubernetes v1.5.2+43a9be4
[root@ip-172-18-0-99 ~]# grep -A 6 networkConfig: /etc/origin/master/master-config.yaml
networkConfig:
  clusterNetworkCIDR: 10.128.0.0/14
  hostSubnetLength: 9
  networkPluginName: redhat/openshift-ovs-subnet
# serviceNetworkCIDR must match kubernetesMasterConfig.servicesSubnet
  serviceNetworkCIDR: 172.30.0.0/16
  externalIPNetworkCIDRs: 
[root@ip-172-18-0-99 ~]# oc get clusternetwork -o yaml
apiVersion: v1
items:
- apiVersion: v1
  hostsubnetlength: 9
  kind: ClusterNetwork
  metadata:
    creationTimestamp: 2017-10-11T17:54:18Z
    name: default
    namespace: ""
    resourceVersion: "458"
    selfLink: /oapi/v1/clusternetworks/default
    uid: 3416f090-aead-11e7-8382-0eff956f0e82
  network: 10.128.0.0/14
  pluginName: redhat/openshift-ovs-subnet
  serviceNetwork: 172.30.0.0/16
kind: List
metadata: {}
resourceVersion: ""
selfLink: ""
[root@ip-172-18-0-99 ~]#

@Weibin
This is a new feature which will be added in 3.7 release. And the old clusterNetworkCIDR and hostSubnetLength will be deprecated in master config.

There is a 3.7 new feature, so 3.5 won't have clusterNetworks in master config. 

And this should be a OCP openshift-ansible bug, since the master config of OCP is generated by openshift ansible:

networkConfig:
  clusterNetworkCIDR: {{ openshift.master.sdn_cluster_network_cidr }}                                                                                                                         
  hostSubnetLength: {{ openshift.master.sdn_host_subnet_length }}
{% if r_openshift_master_use_openshift_sdn or r_openshift_master_use_nuage or r_openshift_master_use_contiv or r_openshift_master_sdn_network_plugin_name == 'cni' %}
  networkPluginName: {{ r_openshift_master_sdn_network_plugin_name_default }}
{% endif %}
# serviceNetworkCIDR must match kubernetesMasterConfig.servicesSubnet
  serviceNetworkCIDR: {{ openshift.common.portal_net }}
  externalIPNetworkCIDRs: {{ openshift_master_external_ip_network_cidrs | default(["0.0.0.0/0"]) | to_padded_yaml(1,2) }}
{% if openshift_master_ingress_ip_network_cidr is defined %}
  ingressIPNetworkCIDR: {{ openshift_master_ingress_ip_network_cidr }}
{% endif %}

Posted PR -- https://github.com/openshift/openshift-ansible/pull/5864

openshift v3.7.0-0.188.0
kubernetes v1.7.6+a08f5eeb62

clusterNetworks already exist in master-config now.

networkConfig:
  clusterNetworkCIDR: 10.128.0.0/14
  clusterNetworks:
  - cidr: 10.128.0.0/14
    hostSubnetLength: 9
  externalIPNetworkCIDRs:
  - 0.0.0.0/0
  hostSubnetLength: 9
  networkPluginName: redhat/openshift-ovs-subnet
  serviceNetworkCIDR: 172.30.0.0/16

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHSA-2017:3188