* Rm py2.7+ code in docker connection plugin
The docker connection plugin was using subprocess.check_output
which only exists in python 2.7 and later. Connection plugins
need to support python2.6 so this replaces it with Popen/communicate()
* Handle docker ver errors in docker connection
Add unit tests for DockerConnection
Fixes#16971
This is because we pass the whole dd command string into the shell
that's running on the contained environment rather than running it
directly from python via subprocess without a shell.
* Remove extraneous imports
* Fix some error handling
* Enable pipelining
* Disable su since it doesn't work
* Add error message when installed docker is not recent enough to
support this plugin
* Move nested functions to class level
* Make transport a class attribute
* Make exec_command, put_file and fetch_file more robust
There doesn't appear to be anything that actually uses tmp_path in the
connection plugins so we don't need to pass that in to exec_command.
That change also means that we don't need to pass tmp_path around in
many places in the action plugins any more. there may be more cleanup
that can be done there as well (the action plugin's public run() method
takes tmp as a keyword arg but that may not be necessary).
As a sideeffect of this patch, some potential problems with chmod and
the patch, assemble, copy, and template modules has been fixed (those
modules called _remote_chmod() with the wrong order for their
parameters. Removing the tmp parameter fixed them.)
