Add TwoPhaseStressMaterial

This adds a material to build a weighted global stress from two phase stresses, switched between by an unconserved order parameter. This material provides properties used by ```StressDivergenceTensors```. The corresponding PR that modified that class to use the new Tensor Mechanics property naming scheme is #4360.

Adresses #4269.
Agree. I copied that from an existing linear_elasticity test. I'll change this and regold.