Congrats! Rapper Bow Wow and girlfriend Erica Mena (star of VH1's Love and Hip Hop New York) are engaged, after just six months of dating. PHOTOS: Celebrity weddings 2014 The musician (real name Shad Moss) confirmed his happy news via Instagram, showing off not only fiancee Mena's new bling, but also his matching diamond-encrusted Rolex watch.…