plugin/hosts provide more configuration flexibility (#2535)

* plugin/hosts provide more configuration flexibility

This patch adds few features to the host plugin
 * no-reverse (both as first argument on the plugin line and inline)
   disable the automatic generation of reserve entries for hosts
 * ttl <duration> (inline only atm)
   allows to change the default ttl (default 5 minutes)
 * reload <duration> (inline only atm)
   allows to change the reloading interval (default 5s)

* plugin/hosts remove superfluous parameters to parse

plugin/hosts/hostsfile_test.go

@@ -12,7 +12,11 @@ import (
 )
 
 func testHostsfile(file string) *Hostsfile {
-	h := &Hostsfile{Origins: []string{"."}}
+	h := &Hostsfile{
+		Origins: []string{"."},
+		hmap:    newHostsMap(),
+		options: newOptions(),
+	}
 	h.parseReader(strings.NewReader(file))
 	return h
 }