title: How to reference a variable in ansible playbook?
body: <p>I'm new to ansible. I can't get the answer in ansible website.
  How to reference a variable in ansible playbook? eg:</p>
  
  <pre><code>- name: Set schedulability
    hosts: oo_first_master
    vars:
      install_nodes: "{{ hostvars
                           | oo_select_keys(groups['oo_nodes_to_config'])
                           | oo_collect('open.common.hostname') }}"
      install_node_vars: "{{ hostvars | oo_select_keys(groups['oo_nodes_to_config']) }}"
  </code></pre>
  
  <p>Seen "install_nodes" refered to like this:</p>
  
  <pre><code>- name: Wait for Node Registration
    command: &gt;
        {{ install.common.client_binary }} get node {{ item | lower }}
    register: omd_get_node
    until: omd_get_node.rc == 0
    retries: 20
    delay: 5
    changed_when: false
    with_items: install_nodes
  </code></pre>
  
  <p>I think reference a variable should be involed with "{{ }}" in ansible playbook. But it's referenced directly. How many way to reference variables in ansible?</p>
  
  <p>Regards</p>