Add replace option to prevent accidental overwrite

[kevin-bates]

This pull request adds a --replace flag to the kernel spec installer CLI tooling to prevent accidental overwrites.

If the directory and kernel.json file already exist, the kernel spec is assumed to exist and, if no --replace option has been included, a error will occur and the process terminated.

$ jupyter k8s-spec install                                                                       
A kernel specification exists at '/usr/local/share/jupyter/kernels/k8s_python' and '--replace' has not been specified.  Terminating.

When --replace is used, the existing directory is copied to a temporary location. If any exceptions are raised when overwriting the existing spec, a "rollback" is performed from the temporary location and a message similar to the following is issued:

An exception was encountered while finalizing the kernel specification and the previous contents have been restored. The exception was: Expecting property name enclosed in double quotes: line 14 column 3 (char 279)