Group and host variables

Note that, in the previous playbook, chapter5_8.yml, we have repeated ourselves in the username and password variables for the two devices under the nexus_devices variable:

    vars:
      nexus_devices: {
        "nx-osv-1": {
          "hostname": "nx-osv-1",
          "username": "cisco",
          "password": "cisco",
          "vlans": [100, 200, 300],
        <skip>
        "nx-osv-2": {
          "hostname": "nx-osv-2",
          "username": "cisco",
          "password": "cisco",
          "vlans": [100, 200, 300],
        <skip>

This is not ideal. If we ever need to update the username and password values, we will need to remember to update at two locations. This increases the management...