Misc cleanups and some fixes for docker connection plugin
Misc cleanups and some fixes for docker connection plugin

* 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

thaim/ansible,thaim/ansible