tier: Add force flag to remove a tier

Sometimes, the tier is down or its address change and we could have many
reasons why it is not possible to reach the tier server during removal
of a tier. For that, we need to add a force flag.

tier.go

@@ -155,12 +155,17 @@ func (adm *AdminClient) EditTier(ctx context.Context, tierName string, creds Tie
 }
 
 // RemoveTier removes an empty tier identified by tierName
-func (adm *AdminClient) RemoveTier(ctx context.Context, tierName string) error {
+func (adm *AdminClient) RemoveTier(ctx context.Context, tierName string, force bool) error {
 	if tierName == "" {
 		return ErrTierNameEmpty
 	}
+
+	queryVals := url.Values{}
+	queryVals.Set("force", strconv.FormatBool(force))
+
 	reqData := requestData{
-		relPath: path.Join(adminAPIPrefix, tierAPI, tierName),
+		relPath:     path.Join(adminAPIPrefix, tierAPI, tierName),
+		queryValues: queryVals,
 	}
 
 	// Execute DELETE on /minio/admin/v3/tier/tierName to remove an empty tier.