Removed ExternalContainerizer.Launch test for disabled Python builds.

Currently ExternalContainerizerTest.Launch relies upon a Python script
which in turn relies upon the Mesos Python egg being available. When
Python is disabled (--disable-python) during the configuration phase,
the Mesos Python egg is not being built, hence the Python script
(test_containerizer.py) can not use it and the test fails.

This patch excludes ExternalContainerizerTest.Launch from the test-suite
when Python has been disabled.

Review: https://reviews.apache.org/r/20823