Add additional description for DNS

[jichenjc]

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.

What this PR does / why we need it:

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
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1. Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

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[sbueringer]

Hm I'm a little bit confused when exactly this property is needed. I wasn't even aware the property exists until now and I did not set it in any of my clusters. (I'm also accessing OpenStack via URL not IP)

[jichenjc]

it's needed. here's the reason :)

the network is created by our logic so if we pass the DNS param, the /etc/resolv.conf of openstack deployed VM will be updated to include the DNS location , my openstack URL is abc.com (different, but like this), not IP ,so when the kubeadm start, because it's openstack cloud provider, it's trying to reach the URL to get additional info, if the DNS is not set, then the URL might not be recognized so that the kubeadm/kubelet is not able to correctly start

[sbueringer]

@jichenjc Never mind. I'm actually setting this property and forgot that I did :)

That explains why it works in my env

/lgtm

[sbueringer]

/test pull-cluster-api-provider-openstack-test