`lxc-ssh.py` removed.

All Playbbooks now user `ssh_lxc` connection.

`ansible_ssh_lxc_name` variable used to specify container name.

Tested and worked correctly with `python==3.8.2` and `ansible==2.9.6` on the
controller and `python==2.7` on the target.

Modification of the stock connection plugin ssh.py to use lxc-attach on
the target host.

We replace any `<cmd>` with
`lxc-attach -n <container_name> /bin/sh -c '<cmd>'`
before sending it through the ssh connection.

Based on the original idea of *Pierre Chifflier* availabe on [GitHub].

The container name should be passed as the `ansible_ssh_lxc_name`
variable.
The `ansible_docker_extra_args` variable is still working for backward
compatibiliy.

ToDo: The docstrings need to be updated, they are still mostly the ones
from ssh.py connection plugin.

We figured out the proper method to access inventory variables (see
README.md in [GitHub]), they need to be propery specified inside the
DOCUMENTATION of the Connection [1], and then they can be obtained with the
`Plugin.get_option()` method. That method should not be called in the
`__init__()`, because options are not yet initialized. Calling it in
`_connect()` returned the correct option.

[GitHub]: https://github.com/chifflier/ansible-lxc-ssh
[1]: https://docs.ansible.com/ansible/2.9/dev_guide/developing_plugins.html

First step to develop a new cleaner ssh_lxc interface.

Here we clone `ansible/plugins/connection/ssh.py` from Ansible version
2.9.6.

It will be adapted to use `lxc-attach` on the target host.