Current version of Ansible (1.7.1) does not merge hashes in the include_vars
task even if told so via hash_behaviour = merge
in your ansible.cfg. ansible/ansible#9116
This action plugin will merge hashes no matter how you have configured your hash_behaviour
.
Save include_vars_merged.py to library/plugins/action/include_vars_merged.py
Save include_vars_merged to library/custom/include_vars_merged