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.

  This commit is a clean up, it don't change anything for the final behaviour.

 Bang on every update is not reliable.

 Now the script on cron contacts re6st registry to collect informations and
 the script itself parameters of the slaves. This will reduce the load in
 and speed up the update of the slave on slapos master.

  * Follow up methods changes on re6stnet registry API.
  * getIPv4Information was simplifed on registry, so reimplement it.
  * Remove scripts generated and keep a single script to orchestate the token management.
  * Drop some code duplication
  * Remove babeld, openvpn and miniupnp as they are not required.
  * Remove useless scripts.
  * Remove re6stnet section, extra-eggs is more them enough and prevent duplications.

We'll probably change the protocol to use msgpack instead of trees of Python
structs. By adding the dependency now, it will be possible to update the NEO
code (which is in development mode) without rebuilding the whole software.