Feat: multiple OpenVPN ciphers for negotiation

- Perfect privacy to accept AES-256-CBC and AES-256-GCM
- Cyberghost default cipher set to AES-256-GCM
- `OPENVPN_CIPHER` accept comma separated cipher values
- Use `ncp-ciphers` for OpenVPN 2.4

internal/provider/custom/openvpnconf_test.go

@@ -32,7 +32,7 @@ func Test_modifyConfig(t *testing.T) {
 			},
 			settings: configuration.OpenVPN{
 				User:      "user",
-				Cipher:    "cipher",
+				Ciphers:   []string{"cipher"},
 				Auth:      "auth",
 				MSSFix:    1000,
 				ProcUser:  "procuser",