Add ability to add MLAG ID to an interface

[mharista]

Example

Port-Channel10
mlag 10

[cathode911]

+1 here, we recently started using ansible for switches provisioning and were surprised that there is no option to configure mlag id on a particular interface. Although imo it should be added to https://github.com/arista-eosplus/ansible-eos-interfaces , this role only configures MLAG between switches

[cathode911]

created issue #26 in arista-eosplus/ansible-eos-interfaces#26