update nuget in distrib test docker files

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