Store kernelspecs under jupyter, not IPython
Add kernel directory relative to sys.prefix
Default to systemwide installation for kernelspecs
Update kernelspec command line interface
Update kernelspec tests
Revert change to test skip condition

This was part of a commit cherry picked from another branch, and this
change didn't make sense on this branch.
Merge pull request #7203 from takluyver/kernelspecs-sys-prefix

Look for kernelspecs relative to sys.prefix