Add --env options to device restart, update commands #109

At the moment we can:

• Specify envvars through edge device start -e ABC=def
• Retain envvars through edge device update
• Retain envvars through edge device restart (by virtue of not removing the original container)

However, in order to alter envvars we have to fully stop the device and reassign them through edge device start using docker inspect to check for (and manually copy) any existing envvars.

Ideally we should have a changeable env when interacting with the device in any way that causes it to run (or continue to run) the app afterwards, so that for instance if I needed to add a new envvar without worrying about the existing ones, I could run edge device restart -e GHI=jkl

Ergo:

1. edge device start -e ABC=def 👉 container has ABC=def
2. edge device restart -e GHI=jkl 👉 container has ABC=def GHI=jkl
3. edge device update -e MNO=pqr 👉 container has ABC=def GHI=jkl MNO=pqr

Removing envvars could subsequently be done in one of two ways: edge device stop && edge device start to start over with a clean slate of envvars, or edge device restart -e ABC= to clear the value of an envvar.