Repiairs SolutionUserObject w/o relying on another libMesh update

This allows for the critical SolutionUserObject fixes to be implemented w/o another libMesh update
(see https://github.com/libMesh/libmesh/pull/307)

In the PR#3591 I replaced the required `nodal_variables` and `elemental_variables` with a single `system_variables` parameter, which if not included loaded all variables in the file. This current change maintains the old syntax, thus does not require the changes made to libMesh.