Add ability to add MLAG ID to an interface

[cathode911]

We recently bumped into issue that no arista-eos ansible roles support configuration of MLAG id on interface. Even more, manual configuration will be overwritten if interfaces role is in use!
IMO this role suits best for MLAG id configuration as it also configures channel-groups etc.

Example:

cat ./group_vars/leafs-rack1.yml
interfaces:
  - name: Ethernet1
    description: sea-hvprod-01-data
    enable: true
    mlag_id: 1

[dathelen]

@cathode911 Thanks for reaching out. Agreed this would be a nice addition to the role. We are planning on looking to addresses in the Q1 2019 timeframe. Let me know if you have any questions.

[cathode911]

@dathelen Just wanted to reach out to check up on timeline, any chance we could get this feature anytime soon?