replace custom merge logic with mergedeep.merge

[mcparaf]

In yaml.py there is logic that is responsible for merging multiple yaml files in order to create a hierarchy of objects that would be then be validated using yamale. This logic enters an infinite loop for this subdict:

switch_link_aggregations:
  - switch_ports:
    - port_id: "7"
      serial: asd
    - port_id: "8"
      serial: qwe

I have replaced the custom merging logic with a call to an external tool (mergedeep) that deals with this case correctly.