MINOR: document how to escape json parameters to ducktape tests

[vvcephei]

• update the readme with an example of correctly escaped JSON to be passed
  as the parameters to a parameterized ducktape tests in docker

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)

[vvcephei]

Getting the escaping right is surprisingly tricky, since the string gets evaluated by Bash, then wrapped in double-quotes to be passed into the Docker command, where it gets evaluated and passed to ducktape.

Since it took me so long to figure it out, I thought it might be nice to add to the README

[mjsax]

Great example. Wondering if it is correct?

--parameters '\''

Should this be

--parameters '\'

because that the end there is '\'

[vvcephei]

Haha, having just spent several hours settling on this exact sequence of characters, I can certify that it's correct ;)

Actually, I would like to get at least one other person to try it out. Do you mind?

To answer your question, here's how it works:
first string (wrapped in single quotes for consistency with the second string): --parameters
first escaped single quote: \'
second string (wrapped in single quotes to avoid evaluating the special characters): {"from_version":"0.10.1.1","to_version":"2.6.0-SNAPSHOT"}
second escaped single quote: \'

Following Bash rules, a sequence of strings and characters just get concatenated, so all of the above gets smashed together to form the desired final string:
--parameters '{"from_version":"0.10.1.1","to_version":"2.6.0-SNAPSHOT"}'

which will then be evaluated again by Bash inside the docker container, so that the final argument to ducktape is the desired argument list: --parameters, followed by {"from_version":"0.10.1.1","to_version":"2.6.0-SNAPSHOT"}.

[ableegoldman]

I tried it out on Friday but left it running and forgot to report back. It passed for me 👍

[ableegoldman]

Much needed, verified that it works for me. Thanks!