YTT lookup starts with the first paths on its command line, which means we
must have the top layer (environment) before the base one if we want the lookup
to find the right overlay of a file.

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