Add support for templated env entries

[kevin-bates]

I've encountered a number of people (including myself) that would like templated environment variable entries in kernelspecs, so I thought I'd implement it add see if this is something useful to the community.

With this change, kernelspec env entries (or even those clients that set extra_env on the KernelManager instance (along with kernel_cmd)) can reference existing env variables using ${ENV_VAR} within various env entries and expect those substitutions to be performed prior to the kernel's launch.

Here's an example kernelspec env entry:

"env": { 
    "PYTHONPATH": "my_special_python_path:${PYTHONPATH}", 
    "FOO": "BAR", 
    "MY_APP_PATH": "my_bin:${PATH}"
}

Following the kernel's invocation, it's environment will include the following variable/values:

PYTHONPATH=my_special_python_path:<notebook server's PYTHONPATH>
FOO=BAR
MY_APP_PATH=my_bin:<notebook server's PATH>

If the templated env variable is not in the launching environment, the templated value will remain in place - just as it shows in the kernelspec's kernel.json file.

Kernelspec administrators should be cognizant of potential to share environments. However, given that's case currently, this shouldn't introduce any new issues.

[blink1073]

👍 for this idea, thanks for proposing it!

[blink1073]

I think a note and warning to admins here would do the trick.

[kevin-bates]

Excellent idea Steven - thank you. I've placed this PR into WIP status since it might be a couple days before I get back to this. Doc updates slipped my mind - thanks again.

[blink1073]

Thanks! I'll leave this open for another day in case anyone has objections.