[oslo] Stepping down from core reviewer

Hi folks,

I no longer have the time to contribute to Oslo in a meaningful way in past few months, due to the company internal stuff, and would like to step down from core reviewer. It was an honor to be one of the great team since 4 years ago. I still work on OpenStack, just have no enough time to focus on Oslo. I hope have more time to contribute again in the future :-)

All the best!