Document how to provide a CA with OpenStack provider

This patch fixes hashicorp#6237 by documenting how to provide a custom CA with
OpenStack provider:
 * Using OS_CACERT environment variable, if available
 * Using cacert_file provider parameter, if configured

website/source/docs/providers/openstack/index.html.markdown

@@ -73,6 +73,10 @@ The following arguments are supported:
 * `insecure` - (Optional) Explicitly allow the provider to perform
  "insecure" SSL requests. If omitted, default value is `false`
 
+* cacert_file - (Optional) Specify custom CA certificate file to check server
+ certificate. If omitted, the `OS_CACERT` environment variable is used. This
+ option has no effects if insecure == true.
+
 * `endpoint_type` - (Optional) Specify which type of endpoint to use from the
  service catalog. It can be set using the OS_ENDPOINT_TYPE environment
  variable. If not set, public endpoints is used.