I suggest that you continue to focus on learning Plone, and the surrounding details of the Linux system as they directly relate to Plone. You might or might not find "a certification" that relates to it, and even if you did, you should bear in mind that "a certification" is not "a Golden Ticket." No one is going to turn you away from a job for not having one, nor give you a job just because you do.
As you continue to learn about Python, Umbuntu, and Plone, you might spend some time looking around at the various certification courses, notice what they do and don't cover, and perhaps you might decide that "it would genuinely help me to know more about this-or-that." If that turns out to be the case, then very seriously studying for that certification might do you a lot of good ... whether or not "getting the corresponding piece of paper to hang on your office wall" also does.
A certification course is a concentrated, scenario-based professional training program which "oh, by the way," leads up to a completion certificate. The process of studying for it, is where the true value-to-you actually lies, whether you do it by home self-study or as part of a training class or both. If you find that merely reading documentation overwhelms you with information without providing you with a clear path to follow through that morass of information, then a certification course might be just what the doctor ordered.