stack/slapos: version up slapos.rebootstrap 3.10 to prevent build loop when zc.buildout version changes.

/reviewed-by: TrustMe

/reviewed-on nexedi/slapos!143

component/keras: version up 2.0.0. load scipy.stats with our own libstdc++ explicitly at the very beginning so that we don't need to build python2 with our won libstdc++.

stack/slapos: use slapos.rebootstrap in any SR but use a dedicated rebootstrap directory in component/slapos only.

because only component/slapos can be build with the different python version than 2.7.

that is the latest version supported by ez_setup.py used in bootstrap-buildout.py.

  This speed up compilation and make thinks much master.

Also, publish connection information to SlapOS

/cc @jerome, @romain, @rafael 

/reviewed-on nexedi/slapos!140

Otherwise, performances get awful on HDD, and not far better on SSD.

  Upgrade officially all SR to latest zc.buildout version.

use slapos.recipe.build:download and slapos.recipe.build:download-unpacked instead of hexagonit.recipe.download.