Introduce better string replacement

[tyBouch]

As it stands our string replacement support for variables is split into two differing calls:
replaceVariable()
and
replaceStoredVariables()

These function slightly different in that one supports testdata. The other does not. This causes issues in items such as API where you cannot use testdata or yml configurations to load dynamic constructed API paths.

As a user I would like these combined with the following load priority:
-Locally set by test or configuration
-Yml file
-Original text if not found