Add shutdown attribute to Interfaces class and update related templates

- Added a `shutdown` boolean attribute to the `Interfaces` class to manage interface states.
- Updated the `eltex.ttp` template to include the `shutdown` command based on the new attribute.
- Modified expected configuration JSON files for various devices to reflect the new `shutdown` attribute, ensuring accurate testing and validation of interface configurations.

tests/fixtures/huawei/config.expected.json

@@ -7,12 +7,14 @@
   "interfaces": [
     {
       "interface": "GigabitEthernet0/0/33",
+      "shutdown": false,
       "ip_address": null,
       "mask": null,
       "description": null
     },
     {
       "interface": "Vlanif120",
+      "shutdown": false,
       "ip_address": "10.26.196.254",
       "mask": 24,
       "description": "SSH"