Should `ipython kernel install` shell out to Jupyter instead of the Python API?

Currently, when you install the IPython kernelspec, it uses the jupyter_client Python API to do it. This ensures that it's using the same Python as IPython, and is part of why we say it doesn't make sense to install kernels in sys.prefix.

If, instead, we shelled out to `jupyter kernelspec install`, then it would be using the 'active' jupyter to decide where to put things. With that, it actually does start to make sense to put kernelspecs in sys.prefix (for envs, at least).