At a retreat for Merrill Lynch financial advisers in a luxury Orlando hotel last month, a group of several dozen men and women in business attire swung their arms back and forth over their heads to Kid Rock’s “All Summer Long” to get their circulation going. The mild aerobics were part of a three-day event orchestrated by Chris Johnson, a wellness guru who has gained influence under John Thiel’s leadership of Bank of America’s Merrill Lynch wealth management business.