Increase python version coverage for tests. (#24762)

* Improve ansible-test inventory handling.
* Fix python 3 re-raise of exception from thread.
* Fix python 3 encoding for windows-integration.
* Run network tests on multiple python versions.
* Run windows tests on multiple python versions.
* Support Shippable delegation using --tox.
* Skip vyos_command on python 3 tests until fixed.
* Add python 3 filtering to local and tox.
* Fix tests to support back to back runs.
* Temporarily test networking with python 2.7 only.

Running the tests back to back causes intermittent test failures
which need to be addressed before we can test multiple versions
in a single test run.

test/runner/tox.ini

@@ -5,5 +5,5 @@ minversion = 2.5.0
 [testenv]
 changedir = {toxinidir}/../../
 commands = {posargs}
-passenv = HOME
+passenv = HOME SHIPPABLE*
 args_are_paths = False