Instead of hardcoding plugin lists in autopath/health, use interfaces. (#1306)

Switched health and autopath plugin to allow any plugins to be used instead
of a hardcoded list. I did not switch federation over since it wasn't
obvious that anything other than kubernetes could be used with it.

Fixes #1291

core/dnsserver/register.go

@@ -167,6 +167,21 @@ func (c *Config) Handler(name string) plugin.Handler {
 	return nil
 }
 
+// Handlers returns a slice of plugins that have been registered. This can be used to
+// inspect and interact with registered plugins but cannot be used to remove or add plugins.
+// Note that this is order dependent and the order is defined in directives.go, i.e. if your plugin
+// comes before the plugin you are checking; it will not be there (yet).
+func (c *Config) Handlers() []plugin.Handler {
+	if c.registry == nil {
+		return nil
+	}
+	hs := make([]plugin.Handler, 0, len(c.registry))
+	for k := range c.registry {
+		hs = append(hs, c.registry[k])
+	}
+	return hs
+}
+
 // groupSiteConfigsByListenAddr groups site configs by their listen
 // (bind) address, so sites that use the same listener can be served
 // on the same server instance. The return value maps the listen