Host Groups may now be associated with a list of puppet classes

or just a singleton puppet class as before.  E.g., this is
now a valid definition in seeds.rb:

    hostgroups = [
        # same as before
        {:name=>"Swift Storage Node",
         :class=>"quickstack::swift::storage"},
        # now mutliple puppet classes work
        {:name=>"Test Pacemaker Qpid",
         :class=>["quickstack::pacemaker::common",
                  "quickstack::pacemaker::qpid"]},
    ]