Update create server process to not add port knocking if Docker to be added later

[jpmckinney]

Right now we run onboarding and core first, which runs the firewall script. The firewall script checks for command -v docker, which will fail. It will then proceed to implement port knocking. https://ocdsdeploy.readthedocs.io/en/latest/deploy/create_server.html#apply-core-changes

We can either:

• Check for either command -v docker or a non-empty DOCKER variable (e.g. add DOCKER to the firewall-settings.local template). That way, we can add a step to the documentation to set DOCKER, if appropriate.
• Document how to reverse the firewall script. I did this by flushing iptables, adding port 22, deploying, and restarting the server. If that's it, we can document the steps more explicitly.

[jpmckinney]

From email discussion: we can instead document how to reset the firewall at the appropriate time.

[jpmckinney]

Fixed by #418.