QA: Enable use of OR conditions in test checks

Until now “TestRunIf” and “TestEnabled” could only handle AND. With this
patch a new class named “Either” is added to “qa_config” and allows OR.
The name “Either” was chosen instead of “Or” as the latter is very close
to the reserved keyword “or”.

Examples:
  ["rapi", Either(["instance-rename", "instance-reboot"])]

  Either(["node-list", "instance-list", "job-list"])

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>