Make MATLAB proxy version configurable on image

[craig-willis]

Problem: Mathworks seems to be updating the proxy about every 2-3 months. v0.3.2 introduces two new environment variables that must be set, which I'm now handling via the image object. I've also introduced the WT_MATLAB_PROXY_VERSION env var via wt_env to supporting configuring the proxy package version going forward without rebuilding.

from girder.plugins.wholetale.models.image import Image
image = Image().findOne({"name": "MATLAB (Desktop) [R2020b]"})
image["config"]["environment"] = ["VERSION=R2020b", "MWI_BASE_URL=/matlab", 
   "MWI_APP_PORT=8888", "WT_MATLAB_PROXY_VERSION=v0.3.2"]
Image().save(image)

To test without using WT:

• Checkout and install https://github.com/whole-tale/repo2docker
• Checkout and install this branch (pip install -e .)
• cd repo2docke_wholetale/examples/matlab
• repo2docker --config /home/ubuntu/repo2docker_wholetale/repo2docker_config.py --user-name jovyan --user-id 1000 --debug --no-run . 2>&1 | grep jupyter-matlab-proxy
• Confirm jupyter-matlab-proxy==v0.3.2
• Change environment.json version to v0.3.1
• Confirm jupyter-matlab-proxy==v0.3.1

To test with WT:

• Build this image and configure in gwvolman/constants.py or just use my craigwillis/repo2docker_wholetale:matlab
• Create and run a MATLAB desktop tale.
• Confirm that v0.3.2 was installed either by inspecting celery logs or package in-container
• Optionally change WT_MATLAB_PROXY_VERSION to v0.3.1 and re-test.