Override StarPU's number of CPU only if provided by Chameleon's user

Fix #92 (closed)

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)