[tests-only] Add feature to be able to run only specific part(s) of test suites in CI

[saw-jan]

This PR adds the feature to be able to run only the certain part(s) of the test suites.

E.g;
ocisIntegrationTests(6, [1, 4]) will run only 1 and 4 parts out of 6

Part of owncloud/QA#670

[phil-davis]

LGTM - that will let anyone choose to run just some pipelines of tests that are split into many parallel parts. That will be handy when trying to investigate some specific test failure(s) - no need to keep re-running all the pipelines.

[phil-davis]

@labkode @ishank011 we have done similar in owncloud repos to make it easy to cut-down CI when someone wants to run just some specific part in order to investigate test fails.