In order to better support use cases relating to containerized environments,
this change adds environment variable support for the `--port` option.  Since
anyone setting a specific port probably doesn't want port retry logic enabled,
the `--port-retries` option has also been backed by an env.

Option `--port` will be backed by env `JUPYTER_PORT` and still defaults to `8888`.
Option `--port-retries` will be backed by env `JUPYTER_PORT_RETRIES` and still
defaults to `50`.

The CLI options will override those set via the envrionment, but environment
values override those set via configuration files.

Closes jupyter#5212