Don't require to shell-escape conf_overrides

[mandre]

Use heredoc syntax to append the value of conf_overrides to the local.conf file. Compared to the previous solution using echo, this removes the need to shell escape your value.

Also remove the useless test, since the default value for inputs.conf_overrides is an empty string which results in a noop.

Fixes #19.

[pierreprinetti]

LGTM :+1: The variable is replaced before Bash is even executed; this means that quotes in the variable are pasted as-is, and could terminate the quotes that start the argument to echo. Good catch @mandre

[EmilienM]

Merged from the beach