Don't add default values to list of variables + some QA

[ewjmulder]

When using default values, the script printed a warning for each default value: "... is not defined and no default is set, replacing by empty". Also when using the -p flag, you can see each default value as a variable with no value.

The cause is a line where each default value is added to the list of variables. This makes no sense to me, so I guess it's a bug. In this PR I removed that and indeed that gets rid of the warning. If there is any reason for that line that I missed, please inform me.

I also included some other small QA changes that I think make sense: fixed a typo, removed trailing whitespace and usage of $() instead of backticks.

Thanks for the great script! Perfect to use in a lean Docker container for some simple templating. Looking forward to your response.

[johanhaleby]

Thanks for the PR. I need to look into this some more before I can answer your question. Stay tuned :)