Register newly modified hosts in new_instances

[goetzk]

This allows following tasks to work with a group of hosts which have been reported as relevant instead of surmising the system state from initial configuration.

[goetzk]

To add some context, I have various tasks like this which use the code

- hosts: cc                                                       
  tags: vm_setup
  tasks:                                                                                       
    - name: Add host to pxetool inventory                                                      
      shell: |                                                                                 
        pxetool set "{{ hostvars[item]['inventory_hostname'] }}" pxemac "{{ hostvars[item]['mac_address'] }}"
        pxetool set "{{ hostvars[item]['inventory_hostname'] }}" pxeip "{{ hostvars[item]['ip_address'] }}"
        pxetool set "{{ hostvars[item]['inventory_hostname'] }}" boot "{{ hostvars[item]['boot_os'] }}"
      when: hostvars[item]['boot_os']|length > 0                                               
      with_items: "{{ groups['new_instances'] }}"                                                                    
    - name: Start VMs
      command: virsh start "{{ item }}"                                                        
      with_items: "{{ groups['new_instances'] }}"                                              
      ignore_errors: yes   

[goetzk]

Since submitting this PR I've begun to generate my new_instances group in a different way, an example of which is below. I may need the code from this PR one day if things change again but for now I will be closing it.

- hosts: localhost
  tags: vm_setup,vm_puppetise
  tasks:
    - name: Fill with systems who's boot_os is specified
      add_host:
        name: "{{ item }}"
        groupname: "new_instances"
      # See also https://www.reddit.com/r/ansible/comments/c1ssl6/hide_skipped_hosts_for_one_task/ for a quieter way
      with_items: "{{ groups['all'] }}"
      when:
        - ( hostvars[item]['boot_os'] |default('')) |length > 0
        - new_instances is not defined