Feature: use a default variable for the recursive include directory

[flybyray]

In several descriptions the default value for include_recursive in the icinga2 configuration will be overriden.
it is considered best practice to not mess with defaults from icinga package configuration files in the conf.d.

if you use a different custom directory the structure remains clean after an update. some backup or rpmnew files can be omitted by this practice. maintenance with tools like rpmconf will be easier.

https://github.com/adfinis-sygroup/ansible-role-icinga2_master/blob/4f9482a03b7eb6bfe867bf9e294d6f8890b4706d/templates/etc/icinga2/icinga2.conf.j2#L62