ansamitocins [ ansamitocin P 3,ansamitocin P 3',ansamitocin P 4 ] maytansinoid antibiotics produced by an actinomycete strain Norcardia sp. No. C-15003 (N-1); P-1 & P-2 identified with maytanacine & maytansinol propionate, respectively; structures for ansamitocins P