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.

oxi/interfaces/models/templates/eltex.ttp

@@ -25,6 +25,7 @@ Active-image: {{ ignore }} {{ _start_ }}
 
 <group name="interfaces">
 interface {{ interface | ORPHRASE }}
+ shutdown {{ shutdown | set(True) }}
  ip address {{ ip_address }} {{ mask | to_cidr }}
  name {{ description | ORPHRASE}}
  description {{ description | ORPHRASE }}