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/node.py

@@ -58,6 +58,8 @@ class Node:
         "category",
         "console_auto_start",
         "netmiko_device_type",
+        "default_username",
+        "default_password",
     ]
 
     def __init__(self, project, compute, name, node_id=None, node_type=None, template_id=None, **kwargs):
@@ -114,6 +116,8 @@ class Node:
         self._first_port_name = None
         self._console_auto_start = False
         self._netmiko_device_type = None
+        self._default_username = None
+        self._default_password = None
 
         # This properties will be recomputed
         ignore_properties = ("width", "height", "hover_symbol")
@@ -222,6 +226,22 @@ class Node:
     def netmiko_device_type(self, val):
         self._netmiko_device_type = val
 
+    @property
+    def default_username(self):
+        return self._default_username
+
+    @default_username.setter
+    def default_username(self, val):
+        self._default_username = val
+
+    @property
+    def default_password(self):
+        return self._default_password
+
+    @default_password.setter
+    def default_password(self, val):
+        self._default_password = val
+
     @property
     def properties(self):
         return self._properties
@@ -844,6 +864,8 @@ class Node:
                 "console_type": self._console_type,
                 "console_auto_start": self._console_auto_start,
                 "netmiko_device_type": self._netmiko_device_type,
+                "default_username": self._default_username,
+                "default_password": self._default_password,
                 "aux": self._aux,
                 "aux_type": self._aux_type,
                 "properties": self._properties,