gather ansible_default_ipv4 if undefined

[extra-vars]

Currently the role assumes the ansible_default_ipv4 variable has been gathered prior to the roles execution. This results in the following failure:

TASK [arillso.hosts : Edits the hosts file in /etc] ****************************************************************************************************
fatal: [piTest]: FAILED! => {"changed": false, "msg": "AnsibleUndefinedVariable: {{ hostvars[inventory_hostname]['ansible_default_ipv4']['address'] }}: 'ansible.vars.hostvars.HostVarsVars object' has no attribute 'ansible_default_ipv4'"}

This PR adds a task to gather only the network facts from the host if ansible_default_ipv4 is undefined.

@arillso thank you for your work.