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>

plugins/connection/lxc.py

@@ -60,7 +60,7 @@ class Connection(ConnectionBase):
     def __init__(self, play_context, new_stdin, *args, **kwargs):
         super(Connection, self).__init__(play_context, new_stdin, *args, **kwargs)
 
-        self.container_name = self._play_context.remote_addr
+        self.container_name = None
         self.container = None
 
     def _connect(self):
@@ -68,12 +68,14 @@ class Connection(ConnectionBase):
         super(Connection, self)._connect()
 
         if not HAS_LIBLXC:
-            msg = "lxc bindings for python2 are not installed"
+            msg = "lxc python bindings are not installed"
             raise errors.AnsibleError(msg)
 
         if self.container:
             return
 
+        self.container_name = self.get_option('remote_addr')
+
         self._display.vvv("THIS IS A LOCAL LXC DIR", host=self.container_name)
         self.container = _lxc.Container(self.container_name)
         if self.container.state == "STOPPED":
@@ -118,7 +120,7 @@ class Connection(ConnectionBase):
         super(Connection, self).exec_command(cmd, in_data=in_data, sudoable=sudoable)
 
         # python2-lxc needs bytes. python3-lxc needs text.
-        executable = to_native(self._play_context.executable, errors='surrogate_or_strict')
+        executable = to_native(self.get_option('executable'), errors='surrogate_or_strict')
         local_cmd = [executable, '-c', to_native(cmd, errors='surrogate_or_strict')]
 
         read_stdout, write_stdout = None, None