Merge variable sub-var-dicts by default.

[Paul-Ferrell]

After talking to Francine, I'm changing the behavior of how sub-variables merge via inheritance.

Given the following configuration:

lvl1:
  variables:
    var: 
      a: A1
lvl2:
  inherits_from: lvl1
   variables:
    var?: 
      a: A2
      b: B2
      c: C2

lvl3:
  inherits_from: lvl2
   variables:
    var: 
      c: C3

lvl4:
  inherits_from: lvl3
   variables:
    var: 
      - b: B4_1
      - b: B4_2

lvl5: 
  inherits_from: lvl4
   variables:
    var: 
      c: C5

We would currently get a single var item with the values {a: A2, b: B2, c: C5} in the test lvl5. This is because setting var in lvl5 would completely replace var from lvl4. For the lvl4 test, we would get two items with b set correctly, but we would lose the host settings for a and the lvl3 settings for c.

What we decided should be the expected behavior is the following:

lvl1 var - {a: A1} # Same as current behavior lvl2 var - {a: A1, b: B2, c: C2} # Same as current. lvl3 var - {a: A1, b: B2, c: C3} # The new setting is overlaid on the old by default. lvl4 var - [{a: A1, b: B4_1, c: C3}, ...] # Each new item is overlaid on the prior item. lvl5 var - [{a: A1, b: B4_1, c: C5}, ...] # The new setting is overlaid on each old item.

If lvl5 had multiple items for var, that would be an error. However, if it used the plus operator var+, that would extend the list, with the new values being overlayed on copies of last item in the old list.

This will also add a new operator: var!. Using this will force the old behavior.