HANG ON HERMITS! LIKE THE first 100 years, one's first Rolls-Royce is generally conceded to be the most difficult to attain.     And despite the foot-long cigar, the glass of bubbly and the proprietorial posture, young Peter Noone is still Rolls-less at the age of 18.     There are, it must be admitted, many sprigs of Peter's age in a similar plight. But he is somewhat different, in that he is already a dollar millionaire, and yesterday signed a £357,000 film deal with M.G.M. which should make him a "real" millionaire by the age of 21.     Peter, in case you haven't guessed, is better known as Herman of the Hermits. He and his group regularly oust the Beatles in America, have sold 10 million records, and have already made their first film, 'Hold On', which will be released soon.     But still the one trapping without which no self-respecting millionaire feels entirely at ease is missing.     Because, as Herman says in what bids fair to become the understatement of the year: "It needs a lot of money to guarantee a Rolls for the rest of your life."