Fix #92

Needs StarPU commit 7b21ab4e50e34f0e92823610f8152d1dfa19b8eb to be effective.

StarPU configuration key `ncpus` is overridden only if Chameleon's user provided a specific value (with `-t` parameter for instance). This allows to keep the value of the environment variable `STARPU_NCPU`, if defined.

(ping @furmento)

See merge request solverstack/chameleon!194