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 = [
    {:name=>"Swift Storage Node",
     :class=>"quickstack::swift::storage"},
    {:name=>"Test Pacemaker Qpid",
     :class=>["quickstack::pacemaker::common",
              "quickstack::pacemaker::qpid"]},
]