fix: address appliance v8 install review findings

- install: resolve the image directory from the version's settings type and
  skip image handling for docker appliances; guard appliance.images
- appliance schema: validate template_properties against template_type,
  align cpu_throttling with the qemu template, add kvm and version idlepc
- conversion: map IOU image to path, kvm disable to accel=tcg, inherit only
  same-type default settings, symbol fallback from the effective category,
  template_properties cannot override structural fields
- allow clearing netmiko_device_type with an empty string
- download the template symbol regardless of the level it is defined at and
  give qemu guests a default symbol

gns3server/schemas/controller/nodes.py

@@ -119,7 +119,7 @@ class NodeBase(BaseModel):
     netmiko_device_type: Optional[str] = Field(
         None,
         description="Device type for Netmiko-based automation tools, overrides the template value",
-        pattern=r"^[a-z0-9_]+$",
+        pattern=r"^[a-z0-9_]+$|^$",
     )
     aux: Optional[int] = Field(None, gt=0, le=65535, description="Auxiliary console TCP port")
     aux_type: Optional[ConsoleType] = None