[occm] update doc for some easy to miss parts (kubernetes#1050)

Author: jichenjc

docs/using-openstack-cloud-controller-manager.md

@@ -43,7 +43,7 @@ The following guide has been tested to install Kubernetes v1.17 on Ubuntu 18.04.
 
 - Bootstrap worker nodes. You need to set `--cloud-provider=external` for kubelet service before running `kubeadm join`.
 
-- Create a secret containing the cloud configuration. You can find an example config file in [`manifests/controller-manager/cloud-config`](https://raw.githubusercontent.com/kubernetes/cloud-provider-openstack/master/manifests/controller-manager/cloud-config). Save the configuration to a file named *cloud.conf*, then:
+- Create a secret containing the cloud configuration. You can find an example config file in [`manifests/controller-manager/cloud-config`](https://raw.githubusercontent.com/kubernetes/cloud-provider-openstack/master/manifests/controller-manager/cloud-config). If you have certs you need put the cert file into folder `/etc/ssl/certs/` and update the update `ca-file` in the configuration file, refer to `ca-file` option [here](https://github.com/kubernetes/cloud-provider-openstack/blob/master/docs/using-openstack-cloud-controller-manager.md#global) for further information. After that, Save the configuration to a file named *cloud.conf*, then:
 
     ```shell
     kubectl create secret -n kube-system generic cloud-config --from-file=cloud.conf
@@ -187,4 +187,4 @@ Although the openstack-cloud-controller-manager was initially implemented with N
 
 ## Exposing applications using services of LoadBalancer type
 
-Refer to [Exposing applications using services of LoadBalancer type](./expose-applications-using-loadbalancer-type-service.md)
+Refer to [Exposing applications using services of LoadBalancer type](./expose-applications-using-loadbalancer-type-service.md)