Instead of having the fetch arguments functions need to be
provided a scope walker to correctly find the right arguments,
which only the internals of the action engine know about
provide a default scope walker (that is the same one the
action engine internal uses) to the storage unit and have it be
the default strategy used so that users need not know how to
pass it in (which they should not care about).
This allows for users to fetch the same mapped arguments as the
internals of the engine will fetch.
Change-Id: I1beca532b2b7c7ad98b09265a0c4477658052d16
5996c8f25e