Refs #4469 : clone 1.1 to 1.2
Fixes #4469: Add empty value variable and overridable variable support.

Make the variables overridable to prevent cfengine spouting warning
lines over variable redeclaration
Merge pull request #290 from VinceMacBuche/ust_3848_implem_4469/handle_empty_variables

Fixes #4469: Add empty value variable support.