Need SRV record support in upstream external-dns

Currently external-dns explicitly ignores SRV records: https://github.com/kubernetes-sigs/external-dns/blob/f763d2a/provider/rfc2136.go#L131

However we need to make the Openshift master-dns-operator work with our DNS environment, i.e. using RFC2136

This will require external-dns to support SRV records for RFC2163. Example records to support for tests: https://github.com/openshift/installer/blob/master/docs/user/metal/install_upi.md#dns-requirements