interface: add ip_mtu parameter with example

[alagoutte]

Tested with 6300, 8xxx and OVA

Some play book example

On vlan :

- hosts: all
  name: Configure ArubaCX
  gather_facts: false
  tasks:
    - name: Configure IP MTU on vlan interface
      arubanetworks.aoscx.aoscx_interface:
        name: vlan23
        state: create
        ip_mtu: 9000

On physical interface (need to be on routing mode)

- hosts: all
  name: Configure ArubaCX
  gather_facts: false
  tasks:
    - name: Configure IP MTU on physical interface
      arubanetworks.aoscx.aoscx_interface:
        name: 1/1/1
        state: create
        ip_mtu: 9000

Close: #38

Need pyaoscx patch too aruba/pyaoscx#35