fix files mode syntax

From Ansible doc (https://docs.ansible.com/ansible/latest/modules/template_module.html#template-module)
For those used to /usr/bin/chmod remember that modes are actually octal numbers. You must either add a leading zero so that Ansible's YAML parser knows it is an octal number (like 0644 or 01777) or quote it (like '644' or '1777') so Ansible receives a string and can do its own conversion from string into number.
Giving Ansible a number without following one of these rules will end up with a decimal number which will have unexpected results.

tasks/elasticsearch-template.yml

@@ -6,7 +6,7 @@
     state: directory
     owner: root
     group: "{{ es_group }}"
-    mode: 2750
+    mode: "2750"
 
 - name: Copy templates to elasticsearch
   copy: src={{ item }} dest={{ es_conf_dir }}/templates owner=root group={{ es_group }} mode=0660