use g5k_charter_time from execo_g5k.planning
respect g5k charter now an option (because best-effort jobs not compatible
with testing queue)
update to latest execo api
- process or action properties instead of accessors
- no more create_dirs option
- less constructor options
- connection instead of connexion
limit bench problem size (16 GB by default)
new nancy cluster graphite: nodes have 256Go RAM. Saturating their RAM
results in too long runs of xpl, default job walltime of 5 hours not long
enough, and the whole cluster bench would take weeks to finish