Add `split_envvar()` method to return key/val or key/None

[JamesAwesome]

In roles.py there are several places where a string representing an envvar (formatted as key=val) is split like so:

key, val = e.split('=', 1)

It's possible however that these strings can also be formatted as simply key due to a nil environment variable being set on the docker container.

eg: if you launch a container like so: docker run -e 'FOO' -it ubuntu when you inspect the container, it's JSON output will look like so:

            "Env": [
                "FOO",
                "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
            ],

The current method of splitting by directly calling .split will cause a stack trace in this event, even if the key which is nil is not something we are looking for.

To prevent that I've simply added a method that will ensure two items are always returned when splitting an envvar string. If an ennvar is nil, the value returned for val will be None.

[ryan-lane]

Thanks for the PR and great catch!

[ryan-lane]

Releasing this into 1.9.0.

[JamesAwesome]

🙇 Thank you!

[ryan-lane]

Released!