Add ZeroTier and Ztncui

See [this thread](https://forum.yunohost.org/t/community-apps-zerotier-virtual-networks/9825)

apps.json

@@ -3079,6 +3079,16 @@
         "state": "working",
         "url": "https://github.com/YunoHost-Apps/zeronet_ynh"
     },
+    "zerotier": {
+        "branch": "master",
+        "category": "system_tools",
+        "revision": "HEAD",
+        "state": "working",
+        "subtags": [
+            "network"
+        ],
+        "url": "https://github.com/YunoHost-Apps/zerotier_ynh"
+    },
     "zomburl": {
         "branch": "master",
         "category": "small_utilities",
@@ -3089,5 +3099,15 @@
             "url_shortener"
         ],
         "url": "https://github.com/courgette/zomburl_ynh"
+    },
+    "ztncui": {
+        "branch": "master",
+        "category": "system_tools",
+        "revision": "HEAD",
+        "state": "working",
+        "subtags": [
+            "network"
+        ],
+        "url": "https://github.com/YunoHost-Apps/ztncui_ynh"
     }
 }