pt-galera-log-explainer: improvments from feedbacks

[ylacancellera]

Don't get scared by +6482 lines, most of it is from adding a regression test (locally generated)
To me commits are short enough, let me know if that's too much.

summary of various changes:

• "grep -a": because some logs are sent in a binary format, even though it's actually readable.
• --skip-merge: useful for cases with identical wsrep node names (got one in actual customer case)
• small refactor of "extractor" sub-logger. It was not using the same params as the general one, it was giving 2 formats of log when using -vv

operators log improvements:

• auto detection because many people missed --pxc-operator
• setting pod name as identifier because it make more sense than using filenames
• enabling a merge mechanism because many customer cases ends up having many pt-k8s-debug-collector with completely different log files when pvc is destroyed. (merge uses podname+deployment+namespace)

Important:

• logs have been added in b15cdf2, but they were generated using a local k3d setup.
• make sure to review 3a5bde7, this is a debatable change. (Multiple commands sharing the same positional arguments alecthomas/kong#408)

If this format of PR is not ideal let me know.

• The contributed code is licensed under GPL v2.0
• Contributor Licence Agreement (CLA) is signed
• Documentation updated
• Test suite update