I\'ve got an Ansible inventory file a bit like this:
[es-masters]
host1.my-network.com
[es-slaves]
host2.my-network.com
host3.my-network.com
[es:children]
es-m
You do it the other way around. You check if the identifier (hostname or IP or whatever is in your inventory) is in the defined group. Not if the group is in the hostvars.
{% if ansible_fqdn in groups['es-masters'] %}
node_master=true
{% else %}
node_master=false
{% endif %}
Provide default in template
# role_name/templates/template.j2
node_master={{ role_name_node_master | default(true) }}
Than override in group_vars
# group_vars/es-masters.yml
role_name_node_master: false