[PR #7369/1bf5a44a backport][stable-6] Fix lxc plugin options (#7370)

Fix lxc plugin options (#7369)

* Fixture for liblxc

Add a fixture to allow testing the lxc connection plugin both with and
without liblxc being present.

Also change the test from unittest to pytest.

* Update liblxc error message

The error is not specific to python2, so remove the version. Also add
a test for it.

* Migrate to options

Because the lxc plugin was only using PlayContext properties, using host
vars like `ansible_lxc_host` didn't work. This is fixed by instead using
the `get_option` method inherited from `AnsiblePlugin`.
The options are not yet available in the `__init__` function, so the
determination of the container name is moved to the `_connect` method,
which is the first time it is actually needed.
The default for the `remote_addr` option is removed, because the string
`inventory_hostname` is not very useful. At all. This seams to have been
spread with copy&paste and a bit of cargo culting. The variable priority
already takes care of setting the value.

* Add changelog fragment

* Fix for Py2.7

`TypeError: super() takes at least 1 argument (0 given)`

* Add plugin type to changelog fragment.

* Restore untemplated default

This partially reverts commit 429d8c8cfb.

---------

Co-authored-by: Felix Fontein <felix@fontein.de>
(cherry picked from commit 1bf5a44a77)

Co-authored-by: Corubba <97832352+corubba@users.noreply.github.com>

changelogs/fragments/7369-fix-lxc-options.yml

@@ -0,0 +1,3 @@
+---
+bugfixes:
+  - lxc connection plugin - properly evaluate options (https://github.com/ansible-collections/community.general/pull/7369).