Allocator capacity conditional breaks on Ansible 2.9

[m-a-leclercq]

Related to #147

This syntax breaks on ansible 2.9. It works on ansible 2.10 (confirmed) and it seems it passed your tests on 2.8.

Minimal example to reproduce behavior :

--
- name: "test"
  hosts: localhost
  connection: local
  tasks:
    - name: Set empty capacity if not defined
      set_fact:
        capacity: "{{ capacity | default('') }}"
    - name: "debug"
      debug:
        msg: /home/elastic/elastic-cloud-enterprise.sh {{ '--capacity ' + capacity if capacity }}

Results on Fedora 35 with Ansible 2.10.15 (python3)

ansible-playbook test.yml 
[WARNING]: No inventory was parsed, only implicit localhost is available
[WARNING]: provided hosts list is empty, only localhost is available. Note that
the implicit localhost does not match 'all'

PLAY [add admin] ***************************************************************

TASK [Gathering Facts] *********************************************************
ok: [localhost]

TASK [Set empty capacity if not defined] ***************************************
ok: [localhost]

TASK [debug] *****************************************************************
ok: [localhost] => 
  msg: '/home/elastic/elastic-cloud-enterprise.sh '

CentOS 7.9 with ansible 2.9.27 (python 2.7.5)

ansible-playbook  test.yml
[WARNING]: provided hosts list is empty, only localhost is available. Note that the implicit localhost does not match 'all'

PLAY [add admin] *************************************************************************************************************

TASK [Gathering Facts] *******************************************************************************************************
ok: [localhost]

TASK [Set empty capacity if not defined] *************************************************************************************
ok: [localhost]

TASK [debug] ***************************************************************************************************************
fatal: [localhost]: FAILED! =>
  msg: |-
    The task includes an option with an undefined variable. The error was: the inline if-expression on line 1 evaluated to false and no else section was defined.

    The error appears to be in '<my_secret_filepath>/test.yml': line 9, column 7, but may
    be elsewhere in the file depending on the exact syntax problem.

    The offending line appears to be:

            capacity: "{{ capacity | default('') }}"
        - name: "debug"
          ^ here

I would suggest using jinja2 syntax to avoid any problems with Ansible 2.9 which I believe is the most widely used version at the moment.
I will create a PR once I check the correct jinja2 syntax.