We’ve seen it happen before, bands announcing a farewell tour only to have a change of heart. And while David Lee Roth recently announced his retirement at the conclusion of his upcoming Las Vegas residency, he did leave himself a window if he reconsiders with a new poster for the run proclaiming: “THE LAST TOUR EVER UNLESS IT ISN’T.”Given Roth’s sense of humor and showmanship, this could simply be a matter of poking fun at…