Add prefix config option which enables more than one server (or instance) to use
a single redis database.  prefix defaults to "waterline" the previous hard coded value
but can be set in options to anything else.  E.g.
 adapter   : 'sails-redis',
  options: {
    parser: 'hiredis',
    prefix: 'waterline1',   // instance prefix to avoid collisions
// ... rest of options