Extract default property value calc into function

Make a function out of the python code that calculates the default value
of a property so that it can be used in multiple places.

An upcoming commit will also use it if it has problems restoring the
property from its backing file.

Change-Id: I22b0f1130e7dce8c4b83e9111ad0d4895637404e
Signed-off-by: Matt Spinler <spinler@us.ibm.com>
1 file changed