Parse args from config files independently

[imcdo]

The Problem

Currently if you add any arguments in your config files that take n arguments, such as exclude, and you have no other arguments, these could potentially get merged with your test path arguments.

For example:
.ducktape/config:

--exclude c/a_test.py, c/b_test.py

and you run:

ducktape c

the arg parse will interpret c as an additional argument to the exclude, so you will now be excluding all tests.

The Solution

Simply parse each set of args independently, then merge them into one combined group of args!

Testing

Unit test added, also tested with our system tests:
Set config to:

--exclude systests/cluster/test_runner_operations.py::SimpleRunnerTest.timeout_test

and executed

ducktape systests/cluster/test_runner_operations.py --max-parallel 9000

which successfully ran our tests, in parallel.

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[stan-is-hate]

Looks good, thanks Ian! Just one comment out of curiosity, but looks good overall and thanks for adding a test.

[stan-is-hate]

why do you need to remove the defaults?

[imcdo]

otherwise the merging doesn't work, as the defaults will override the pervious files contributions. (the parser will always include the defaults when parsing the args)