Add external-dns support for openstack

[networkhell]

/kind feature

1. Describe IN DETAIL the feature/behavior/change you would like to see.

Currently external-dns as a replacement for dns-controller seems to be only available for the following cloud providers: GCP and AWS

It should not be a big deal to implement this for openstack, too.

2. Feel free to provide a design supporting your feature request.

I did not dig too deep into the code but maybe it could be as simple as this:

diff upup/pkg/fi/cloudup/template_functions.go upup/pkg/fi/cloudup/template_functions_patched.go
821a822,823
> 	case kops.CloudProviderOpenstack:
> 		argv = append(argv, "--provider=designate")

[hakman]

/assign @zetaab