use of defaults inside role

this is to use Juniper-related playbooks with CLAB. Placing this var inside defaults should be very safe, as defaults inside role gets the least priority.

geant/gap_ansible/roles/base_config/defaults/main.yml

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 # defaults file for base_config
+# ansible_net_has_2RE: false  # safe to set because when ansible gets this var from the real Juniper box, it will be overwritten.