jinja2 default filter

In order to provide a default value, use this filter in jinja2 template.
Example:

...
  bind {{ haproxy_listen_address }}:{{ haproxy_listen_port|default('80') }}
...
{% if haproxy_stats|default(True) %}
...

If the variable haproxy_listen_port or haproxy_stats is not defined, the ansible will not cause an error.