set attr from interfaces.yml

[obedouet]

I propose a little modification to allow setting attributes like MTU from interfaces.yml on a device where interfaces are templated.

Example:
device_types.yml:

- model: TOR-8P
  manufacturer: GNS3
  part_number: vlab-eos
  slug: tor-8p
  interfaces:
    - name: Ethernet1
      type: 1000base-t
      description: UPLINK

devices.yml:

- name: gns3-tor
  device_type: TOR-8P

interfaces.yml:

- name: Ethernet1
  mtu: 9100
  device: gns3-tor

[tobiasge]

Please extend the yml files with examples.