The DOCKER_TEST_DEBUG environment variable can be used to
enable executor debug mode.

Use the VERBOSE_TEST_LOGGING environment variable to
enable verbose logging for tests.

This reverts commit a6ab4867, reversing
changes made to 5c7900bc.

Bogus test phase.

This by default disable tests that requires the capabilities
to create new docker network on the fly.

Also refactor related tests to avoid spending time discovering
environments when the test doesn't need it.

All python files except from scripts are not executable
anymore.

This will allow to run several tests in parallel.

This reverts commit 85cef289.

This allows to build the error message as expected with both
Python 2 and 3.