Famed keyboard player Chuck Leavell is — as a native Georgian — all about living amid the moss, even if the group he’s most associated with, the Rolling Stones, are by definition averse to it. A new documentary opening this weekend, “Chuck Leavell: The Tree Man,” charts his double life, as one of the most renowned session musicians and touring sidemen in rock ‘n’ roll on one hand, and a celebrated environmentalist with a fixation on Southern forestry on the other.