* Britain, Luxembourg, HK among new issuers this year* Next year momentum swinging to emerging mkt issuers* Tunisia, Kenya among those considering sukuk* Drop in oil price may prompt new Gulf issuesBy Karin StroheckerLONDON, Dec 2 (Reuters) - This year&aposs rush by top-rated non-Muslim countries to tap the burgeoning Islamic finance market may not be repeated next year but a new crop of sovereign