plugin/kubernetes: use struct{} was map values (#2125)
plugin/kubernetes: use struct{} was map values (#2125)

This takes less space than a bool, the value isn't used for anything
else than "present in map" any way.

Signed-off-by: Miek Gieben <00b357f94cf3a1145a32703fb42e2d80418ab806@miek.nl>
yongtang/coredns,jremond/coredns,coredns/coredns,miekg/coredns,johnbelamaric/coredns,jremond/coredns,ii/coredns,ii/coredns,chrisohaver/coredns,johnbelamaric/coredns,miekg/coredns