He is either the leading man of MSNBC or its leading agent provocateur, but Keith Olbermann has no problem embracing either role. “You can’t spell momentum without Olbermann — or something like that,” he said in a telephone interview, with a typical sprinkle of wry in his voice. The momentum reference related to MSNBC’s recent aggressive positioning of the program “Countdown With Keith Olbermann” as the centerpiece of this all-news cable network’s latest effort to become more competitive with Fox News Channel and CNN. MSNBC revamped its prime-time schedule two weeks ago, shelving many of its prime-time hosts in favor...