cleanup distribtest Dockerimages

tools/dockerfile/distribtest/csharp_centos7_x64/Dockerfile

tools/dockerfile/distribtest/csharp_jessie_x64/Dockerfile

tools/dockerfile/distribtest/csharp_jessie_x86/Dockerfile

...s/dockerfile/distribtest/csharp_ubuntu1404_x64/Dockerfile

...s/dockerfile/distribtest/csharp_ubuntu1504_x64/Dockerfile

...s/dockerfile/distribtest/csharp_ubuntu1510_x64/Dockerfile

...s/dockerfile/distribtest/csharp_ubuntu1604_x64/Dockerfile

tools/dockerfile/distribtest/csharp_wheezy_x64/Dockerfile

tools/dockerfile/distribtest/node_centos7_x64/Dockerfile

tools/dockerfile/distribtest/node_jessie_x64/Dockerfile

tools/dockerfile/distribtest/node_jessie_x86/Dockerfile

tools/dockerfile/distribtest/node_ubuntu1204_x64/Dockerfile

tools/dockerfile/distribtest/node_ubuntu1404_x64/Dockerfile

tools/dockerfile/distribtest/node_ubuntu1504_x64/Dockerfile

tools/dockerfile/distribtest/node_ubuntu1510_x64/Dockerfile

tools/dockerfile/distribtest/node_ubuntu1604_x64/Dockerfile

tools/dockerfile/distribtest/python_arch_x64/Dockerfile

tools/dockerfile/distribtest/python_centos6_x64/Dockerfile

tools/dockerfile/distribtest/python_centos7_x64/Dockerfile

tools/dockerfile/distribtest/python_fedora20_x64/Dockerfile