Fix CONDITIONAL_BARE_VARS warning

[JoyceBabu]

Fixes the following warning when running the role

TASK [monit : config - Setup mail alerts] *******************************************************************************************************************************************
task path: /home/sysadmin/ansible-playbook/roles/monit/tasks/config.yml:26
[DEPRECATION WARNING]: evaluating monit_mail_enabled as a bare variable, this behaviour will go away and you might need to add |bool to the expression in the future. Also see
CONDITIONAL_BARE_VARS configuration toggle.. This feature will be removed in version 2.12. Deprecation warnings can be disabled by setting deprecation_warnings=False in
ansible.cfg.