Move cfg.get try/excepts to safe_git

Moved cfg.get's in try / except blocks to safe_get

Move safe_get to conf.CephConf
Update existing usage of safe_get
Update try / excepts to safe_get
Update tests in test_mon
Update test in mon.catch_mon_errors to except None or not in mon_initial_members better
Add test for both states of mon_initial_members