nodes: per-node default credentials seeded from the template

Add default_username/default_password as controller-only node properties
(the netmiko_device_type pattern): they are not sent to the compute,
persist with the project topology and can be updated or cleared per
node. Creating a node from a template seeds them from the template
appliance metadata, and the metadata itself is dropped there so it
never leaks into the node properties.

gns3server/controller/project.py

@@ -584,6 +584,12 @@ class Project:
         default_name_format = template.pop("default_name_format", "{name}-{0}")
         if name is None:
             name = default_name_format.replace("{name}", template_name)
+        # the appliance metadata stays template level: only the default
+        # credentials are seeded on the node (where they can be overridden)
+        appliance_metadata = template.pop("appliance_metadata", None) or {}
+        for field in ("default_username", "default_password"):
+            if appliance_metadata.get(field):
+                template[field] = appliance_metadata[field]
         node_id = str(uuid.uuid4())
         node = await self.add_node(compute, name, node_id, node_type=node_type, **template)
         return node