PETSc Krylov solver nonzero guess logic seems buggy

[Jan Blechta]

If provided solution vector is not initialized, nonzero guess is set to false, see https://bitbucket.org/fenics-project/dolfin/src/241684bbf19fc2dd9d035d1e7c1d16a14dfeade2/dolfin/la/PETScKrylovSolver.cpp?at=master&fileviewer=file-view-default#PETScKrylovSolver.cpp-310

1. Why don't we just keep PETSc's default?
2. This overrides the parameter or what user set using set_nonzero_guess.

[Jan Blechta]

Fixed by e286fbec945bd31111cac57329f424250edde97a.