Removed ExternalContainerizer.Launch test for disabled Python builds.
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

dforsyth/mesos,abudnik/mesos,neilconway/mesos,zmalik/mesos,andschwa/mesos,kaysoky/mesos,Aman-Jain-14/customizedMesos,neilconway/mesos,Aman-Jain-14/customizedMesos,Gilbert88/mesos,dforsyth/mesos,gsantovena/mesos,verizonlabs/mesos,reneploetz/mesos,dforsyth/mesos,zmalik/mesos,zmalik/mesos,Gilbert88/mesos,verizonlabs/mesos,jpeach/mesos,Aman-Jain-14/customizedMesos,abudnik/mesos,zmalik/mesos,chhsia0/mesos,dforsyth/mesos,jpeach/mesos,abudnik/mesos,yuquanshan/customizedMesos,verizonlabs/mesos,yuquanshan/customizedMesos,craimbert/mesos,andschwa/mesos,neilconway/mesos,gsantovena/mesos,Gilbert88/mesos,zmalik/mesos,chhsia0/mesos,abudnik/mesos,reneploetz/mesos,kaysoky/mesos,kaysoky/mesos,jpeach/mesos,gsantovena/mesos,Gilbert88/mesos,craimbert/mesos,verizonlabs/mesos,asamerh4/mesos,dforsyth/mesos,verizonlabs/mesos,asamerh4/mesos,reneploetz/mesos,abudnik/mesos,craimbert/mesos,reneploetz/mesos,craimbert/mesos,andschwa/mesos,neilconway/mesos,Gilbert88/mesos,kaysoky/mesos,Gilbert88/mesos,gsantovena/mesos,andschwa/mesos,andschwa/mesos,craimbert/mesos,gsantovena/mesos,asamerh4/mesos,jpeach/mesos,chhsia0/mesos,yuquanshan/customizedMesos,yuquanshan/customizedMesos,reneploetz/mesos,gsantovena/mesos,yuquanshan/customizedMesos,Aman-Jain-14/customizedMesos,jpeach/mesos,asamerh4/mesos,craimbert/mesos,dforsyth/mesos,reneploetz/mesos,dforsyth/mesos,zmalik/mesos,Aman-Jain-14/customizedMesos,jpeach/mesos,Aman-Jain-14/customizedMesos,verizonlabs/mesos,chhsia0/mesos,jpeach/mesos,abudnik/mesos,craimbert/mesos,abudnik/mesos,neilconway/mesos,andschwa/mesos,neilconway/mesos,kaysoky/mesos,yuquanshan/customizedMesos,andschwa/mesos,yuquanshan/customizedMesos,asamerh4/mesos,zmalik/mesos,Aman-Jain-14/customizedMesos,reneploetz/mesos,verizonlabs/mesos,kaysoky/mesos,chhsia0/mesos,chhsia0/mesos,Gilbert88/mesos,gsantovena/mesos,asamerh4/mesos,chhsia0/mesos,asamerh4/mesos,neilconway/mesos,kaysoky/mesos