pythonPackages.keystoneclient: Disable tests

````
run_tests.sh: interpreter directive changed from "/bin/bash" to "/nix/store/nyj6xd7s1n1w8c0xdwk5ddhi7bjcyi9x-bash-4.3-p46/bin/bash"
No virtual environment found...create one? (Y/n) builder for ‘/nix/store/qcrhq2f7llvzyc37ili94ff50z7vlgn3-python2.7-keystoneclient-1.8.1.drv’ failed with exit code 1
error: build of ‘/nix/store/qcrhq2f7llvzyc37ili94ff50z7vlgn3-python2.7-keystoneclient-1.8.1.drv’ failed
````