future-proof bare variables

An install run now outputs the following:

TASK [wordpress-init : Install openid connect plugin] **************************
[DEPRECATION WARNING]: evaluating 'WP_OPENID_CONNECT_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.
changed: [localhost]

TASK [wordpress-init : Set openid connect plugin options] **********************
[DEPRECATION WARNING]: evaluating 'WP_OPENID_CONNECT_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.
changed: [localhost]

TASK [wordpress-init : Extend functions.php file] ******************************
[DEPRECATION WARNING]: evaluating 'WP_OPENID_CONNECT_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.
skipping: [localhost]

TASK [wordpress-init : Copy additional functions file] *************************
[DEPRECATION WARNING]: evaluating 'WP_OPENID_CONNECT_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.
skipping: [localhost]

We should make sure this works in future versions.