Improve performance of hiera config stability check

The check to assert that a hiera configuration is stable with respect to interpolated values, does not take into account that a global variable (a variable that starts with '::') is immutable once set.

The performance of this check will be greatly improved for use-cases that have a hiera.yaml files similar to this example file from a comment in PUP-7294 if such variables are exempt from the check.