plugin/kubernetes: smaller map (#2383)

* plugin/kubernetes: smaller map

to continue with a theme: the map[string]bool can be reduced
to map[string]struct{} to reduce a tiny amount of memory.

Signed-off-by: Miek Gieben <miek@miek.nl>

* fix tests

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/kubernetes/setup.go

@@ -187,7 +187,7 @@ func ParseStanza(c *caddy.Controller) (*Kubernetes, error) {
 			args := c.RemainingArgs()
 			if len(args) > 0 {
 				for _, a := range args {
-					k8s.Namespaces[a] = true
+					k8s.Namespaces[a] = struct{}{}
 				}
 				continue
 			}