Allow to set extra variables on enoslib hosts

Can be useful to be able to set variables like ip: https://kubespray.io/#/docs/ansible/vars

Example to use the IP address of the second interface of the node (really untested, this is just the idea):

  resources:
    machines:
      - roles: [kube_node, k8s_cluster]
        cluster: paravance
        nodes: 1
        min: 1
        extra_vars:
          ip: '{{host_vars[network_interfaces[1][address]]}}'

Requires enoslib#205 (not yet available in enoslib)