Getter for mapped paths from KeyValueTreeTransform

Getter for mapped paths from KeyValueTreeTransform

This allows identifying which values the transform will use.  In the
long run, this may be replaced with another mechanism (or removed
altogether), but for now, this makes it easier to do piecewise
transition from old mdp parsing to using KeyValueTree and options.

Currently, the values are returned in alphabetical order, but subsequent
work may need this in the order the rules are added.  That will be easy
to change internally to the transform.

Change-Id: I3f225ce4f693644e5ac17685f0a887eec02807a8