connection: ssh: Clear environment when connecting to LXC containers

We should clear the environment before connecting to an LXC container to
avoid inheriting host variables that may break container services like
the following failure in rabbitmqctl:

TASK [Get status of rabbitmq] *************************************************************************************************************************************************************************************
fatal: [container1]: FAILED! => {"changed": true, "cmd": ["rabbitmqctl", "status"], "delta": "0:00:00.705116", "end": "2017-06-06 20:03:13.771796", "failed": true, "rc": 69, "start": "2017-06-06 20:03:13.066680", "stderr": "Error: unable to connect to node 'rabbit@vagrant-openSUSE-Leap': nodedown\n\nDIAGNOSTICS\n===========\n\nattempted to contact: ['rabbit@vagrant-openSUSE-Leap']\n\nrabbit@vagrant-openSUSE-Leap:\n  * unable to connect to epmd (port 4369) on vagrant-openSUSE-Leap: address (cannot connect to host/port)\n\n\ncurrent node details:\n- node name: 'rabbitmq-cli-82@localhost'\n- home dir: /var/lib/rabbitmq\n- cookie hash: NqWRA5RzO5daz4Jb5LJsXg==", "stderr_lines": ["Error: unable to connect to node 'rabbit@vagrant-openSUSE-Leap': nodedown", "", "DIAGNOSTICS", "===========", "", "attempted to contact: ['rabbit@vagrant-openSUSE-Leap']", "", "rabbit@vagrant-openSUSE-Leap:", "  * unable to connect to epmd (port 4369) on vagrant-openSUSE-Leap: address (cannot connect to host/port)", "", "", "current node details:", "- node name: 'rabbitmq-cli-82@localhost'", "- home dir: /var/lib/rabbitmq", "- cookie hash: NqWRA5RzO5daz4Jb5LJsXg=="], "stdout": "Status of node 'rabbit@vagrant-openSUSE-Leap' ...", "stdout_lines": ["Status of node 'rabbit@vagrant-openSUSE-Leap' ..."]}
	to retry, use: --limit @/vagrant/tests/test-rabbitmq-server-functional.retry

The reason for this failure is that the HOSTNAME variable is being
inherited by the host (vagrant-openSUSE-Leap) and the rabbitmqctl
command uses this variable to guess the host it should try to
connect to.

This is similar to what the upstream lxc connection module is doing.

Link: https://github.com/ansible/ansible/blob/devel/lib/ansible/plugins/connection/lxc.py#L121
Change-Id: Ic3b8bf9a9aaf8738c49490e22fec2fc0b5140ac0

connection/ssh.py

@@ -65,7 +65,7 @@ class Connection(SSH.Connection):
         """run a command on the remote host."""
 
         if self._container_check():
-            lxc_command = 'lxc-attach --name %s' % self.container_name
+            lxc_command = 'lxc-attach --clear-env --name %s' % self.container_name
             cmd = '%s -- %s' % (lxc_command, cmd)
 
         if self._chroot_check():