2.1: update test_imagestream_sample test for 2.1
2.1: update test_imagestream_sample test for 2.1

redhat-developer/s2i-dotnetcore,omajid/s2i-dotnetcore,omajid/s2i-dotnetcore,redhat-developer/s2i-dotnetcore,omajid/s2i-dotnetcore,redhat-developer/s2i-dotnetcore