Petsc: SetUniformValue() didn't properly call UpdateGhosts()

I have essentially waved out few years ago the need to call explicitly UpdateGhosts(): the rationale that it should be called by default whenever an operation needs it, except if the user know exactly what he does and elect not to do it (might be the case for instance when several operations are performed in a row upon vectors and matrices: we might want to call the update only once at the end).

But it was not done in SetUniformValues() whereas it should have been - as I discovered while trying a test in parallel in #1707 which inexplicably failed, until I find out this miss.