Fix for deprecation warning 'set_available_variables is being deprecated'

[epicanthal]

Fix for [DEPRECATION WARNING]: set_available_variables is being deprecated. Use "@available_variables.setter" instead.. This feature will be removed in version 2.13.

Reference: https://github.com/ansible/ansible/pull/55435

Tested with Ansible 2.9.7

[goneri]

recheck

[softwarefactory-project-zuul[bot]]

Build failed.

• ansible-tox-py36 : TIMED_OUT in 1h 01m 36s
• ansible-tox-py38 : FAILURE in 8m 00s
• ansible-tox-py39 : FAILURE in 8m 18s
• ansible-tox-linters : FAILURE in 7m 39s
• ansible-tox-py27 : FAILURE in 12m 44s