Update 20_setupextrasvars.rake for Build object

This commit updates the loading of extras vars in lieu of the new build
object architecture. This allows us to read in data from the extras
files as needed without worrying if specific parameters are defined or
not. The currently 'overrideable' params are still kept as such to make
sure we can still override as necessary.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>