* 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
Also making PlayContext a child class of the Playbook Base class,
which gives it access to all of the FieldAttribute code to ensure
field values are correctly typed after post_validation
Fixes#11381
