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/schemas/controller/nodes.py

@@ -121,6 +121,14 @@ class NodeBase(BaseModel):
         description="Device type for Netmiko-based automation tools, overrides the template value",
         pattern=r"^[a-z0-9_]+$|^$",
     )
+    default_username: Optional[str] = Field(
+        None,
+        description="Default username to log into the node, seeded from the template appliance metadata",
+    )
+    default_password: Optional[str] = Field(
+        None,
+        description="Default password to log into the node, seeded from the template appliance metadata",
+    )
     aux: Optional[int] = Field(None, gt=0, le=65535, description="Auxiliary console TCP port")
     aux_type: Optional[ConsoleType] = None
     properties: Optional[dict] = Field(default_factory=dict, description="Properties specific to an emulator")