Simplify check of principals in a case insensitive way

Fixes #679

authority/provisioner/sign_ssh_options.go

@@ -454,20 +454,12 @@ func containsAllMembers(group, subgroup []string) bool {
 	if lsg > lg || (lg > 0 && lsg == 0) {
 		return false
 	}
-	groupLower := []string{}
-	subgroupLower := []string{}
-	for _, s := range group {
-		groupLower = append(groupLower, strings.ToLower(s))
-	}
-	for _, s := range subgroup {
-		subgroupLower = append(subgroupLower, strings.ToLower(s))
-	}
 	visit := make(map[string]struct{}, lg)
 	for i := 0; i < lg; i++ {
-		visit[groupLower[i]] = struct{}{}
+		visit[strings.ToLower(group[i])] = struct{}{}
 	}
 	for i := 0; i < lsg; i++ {
-		if _, ok := visit[subgroupLower[i]]; !ok {
+		if _, ok := visit[strings.ToLower(subgroup[i])]; !ok {
 			return false
 		}
 	}