Fix config_string i18n

The prior `en.yml` localization of the `-e` "config_string" flag contained invalid configuration text; i.e. if one were to insert the specified defaults into the `-e` flag, Logstash would produce an error.

This PR relocates those defaults to a constant and then references those constants in the `I18n` invocation, ensuring that they will stay in sync for the future.

As a minor aside, the way the English reads, it seems that because config_string has defaults that one could invoke `bin/logstash agent` with no arguments and it will use the defaults. But that is not the actual behavior. So I also updated the text to indicate to the user if they want to just run logstash in "default mode", they must provide an empty string to the `-e` flag.

Also, while I was in the `en.yml` file I fixed a missing localization which produced the following text when invoking `agent --help`: `translation missing: en.logstash.agent.flag.configtest`. As is mentioned in the commit message, I recognize that is likely not the desired English text, but it is surely better than the error text.

*I realize it is bad form to include two different purposed commits in a single PR, but the missing key change is so small I wouldn't think you'd want a separate PR for it.*