Provide a way to merge/join lists within a HOT template

Currently lists are used within HOT Templates for many things. Yet, there is no way to merge/join multiple lists into a single list.

e.g.,
parameters:
  list1:
    default: [ "a", "b" ]
  list2:
    default: [ "x", "y", z" ]

resources:
  blah:
    properties:
      newlist: { get_param list1 } + { get_param list2 }