Use a custom inventory so the test does not depend on "host" fixture and
run faster.
Also use AnsibleRunner instead of AnsibleBackend.

This will be used in next commit.

Following specification on #431

* Use ansible-inventory cli to parse inventory, group and host variables

* Ansible backend re-use existing backend (local, ssh) to run commands.
  This is a breaking change because we do not support all connection
  backends from ansible.

* The Ansible module run with the ansible cli.

* Add more tests

* Fix the skipped "encoding" test which pass now.

This fix maintainability issue with ansible and the license issue
(ansible is GPL).

Closes #431

Group can be nested with the ":children" notation.
https://docs.ansible.com/ansible/latest/user_guide/intro_inventory.html#groups-of-groups-and-group-variables

Also handle "all" like a normal group since "ansible-inventory" output
all groups (including "ungrouped") to be a children of "all".