Add additional description for DNS (#497)

I am working on my env and noticed that the new VM is not able to
start because my VM can't talk to openstack URL (which is web URI
instead of IP), so we need guide user to create this DNS explicility.

api/v1alpha2/openstackcluster_types.go

@@ -50,6 +50,8 @@ type OpenStackClusterSpec struct {
 	Subnet SubnetFilter `json:"subnet,omitempty"`
 
 	// DNSNameservers is the list of nameservers for OpenStack Subnet being created.
+	// Set this value when you need create a new network/subnet while the access
+	// through DNS is required.
 	DNSNameservers []string `json:"dnsNameservers,omitempty"`
 	// ExternalRouterIPs is an array of externalIPs on the respective subnets.
 	// This is necessary if the router needs a fixed ip in a specific subnet.