provide options to desactivate IO cache in order to speed up build time

[npf]

• option 1: use eatmydata around the kameleon build process: eatmydata kameleon build ....
  -> add to documentation

• option 2: add eatmydata in the relevant steps, for sub commands used by kameleon

• option 3: eatmydata may not work in some cases (e.g. when using the last version of virtualbox) -> use a systemtap trick to change the system calls.
  -> see grid5000/environments-recipes@4957d5b
  This may not be desirable in the local context (global change for the system), so limit to the out context if it is a VM.