Implement variable overlay with dotted paths

If an overlay variable has a dotted name, it is used as a path to patch
a nested value in the original variable.

For instance, given the following original variables:

variables:
  d:
    a: value
    b: other

A config overlay can change the 'a' value without touching the other 'd'
attributes :

variables:
  d.a: new value


The resulting variables would be :

variables:
  d:
    a: new value
    b: other