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.

tests/unit/plugins/connection/test_lxc.py

@@ -71,3 +71,16 @@ class TestLXCConnectionClass():
 
         with pytest.raises(AnsibleError, match='lxc python bindings are not installed'):
             conn._connect()
+
+    def test_remote_addr_option(self):
+        """Test that the remote_addr option is used"""
+        play_context = PlayContext()
+        in_stream = StringIO()
+        conn = connection_loader.get('lxc', play_context, in_stream)
+
+        container_name = 'my-container'
+        conn.set_option('remote_addr', container_name)
+        assert conn.get_option('remote_addr') == container_name
+
+        conn._connect()
+        assert conn.container_name == container_name