libnetwork: refactor rule management to use Ensure method for Append and Insert operations

Signed-off-by: Andrey Epifanov <aepifanov@mirantis.com>

daemon/libnetwork/iptables/iptables.go

@@ -467,22 +467,25 @@ func (r Rule) WithChain(chain string) Rule {
 	return wc
 }
 
-// Append appends the rule to the end of the chain. If the rule already exists anywhere in the
+// ensure appends/insert the rule to the end of the chain. If the rule already exists anywhere in the
 // chain, this is a no-op.
-func (r Rule) Append() error {
+func (r Rule) ensure(op Action) error {
 	if r.Exists() {
 		return nil
 	}
-	return r.exec(Append)
+	return r.exec(op)
+}
+
+// Append appends the rule to the end of the chain. If the rule already exists anywhere in the
+// chain, this is a no-op.
+func (r Rule) Append() error {
+	return r.ensure(Append)
 }
 
 // Insert inserts the rule at the head of the chain. If the rule already exists anywhere in the
 // chain, this is a no-op.
 func (r Rule) Insert() error {
-	if r.Exists() {
-		return nil
-	}
-	return r.exec(Insert)
+	return r.ensure(Insert)
 }
 
 // Delete deletes the rule from the kernel. If the rule does not exist, this is a no-op.