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/eltex/config.expected.json

@@ -7,30 +7,35 @@
   "interfaces": [
     {
       "interface": "TenGigabitEthernet1/0/2",
+      "shutdown": true,
       "ip_address": null,
       "mask": null,
       "description": "FREE"
     },
     {
       "interface": "TenGigabitEthernet1/0/11",
+      "shutdown": true,
       "ip_address": null,
       "mask": null,
       "description": "FREE"
     },
     {
       "interface": "vlan 1700",
+      "shutdown": false,
       "ip_address": "13.36.8.1",
       "mask": 24,
       "description": "sw-test_HW"
     },
     {
       "interface": "vlan 666",
+      "shutdown": false,
       "ip_address": null,
       "mask": null,
       "description": "test"
     },  
     {
       "interface": "vlan 777",
+      "shutdown": false,
       "ip_address": null,
       "mask": null,
       "description": "test2"