Maint: refactor VPN configuration structure

- Paves the way for Wireguard
- VPN struct contains Type, Openvpn and Provider configurations
- OpenVPN specific options (e.g. client key) moved from Provider to Openvpn configuration struct
- Move Provider configuration from OpenVPN configuration to VPN
- HTTP control server returns only openvpn settings (not provider settings)

internal/configuration/openvpn_test.go

@@ -13,9 +13,6 @@ func Test_OpenVPN_JSON(t *testing.T) {
 	in := OpenVPN{
 		Root:  true,
 		Flags: []string{},
-		Provider: Provider{
-			Name: "name",
-		},
 	}
 	data, err := json.MarshalIndent(in, "", "  ")
 	require.NoError(t, err)
@@ -28,35 +25,10 @@ func Test_OpenVPN_JSON(t *testing.T) {
   "run_as_root": true,
   "cipher": "",
   "auth": "",
-  "provider": {
-    "name": "name",
-    "server_selection": {
-      "tcp": false,
-      "regions": null,
-      "groups": null,
-      "countries": null,
-      "cities": null,
-      "hostnames": null,
-      "names": null,
-      "isps": null,
-      "owned": false,
-      "custom_port": 0,
-      "numbers": null,
-      "encryption_preset": "",
-      "free_only": false,
-      "stream_only": false
-    },
-    "extra_config": {
-      "encryption_preset": "",
-      "openvpn_ipv6": false
-    },
-    "port_forwarding": {
-      "enabled": false,
-      "filepath": ""
-    }
-  },
   "custom_config": "",
-  "version": ""
+  "version": "",
+  "encryption_preset": "",
+  "ipv6": false
 }`, string(data))
 	var out OpenVPN
 	err = json.Unmarshal(data, &out)