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

This reverts commit cd33366e.

MariaDB Connector/C 2.3.2 is broken. Here is strace output while running
test_max_allowed_packet from NEO test suite:

poll([{fd=4, events=POLLIN|POLLPRI}], 1, 0) = 0 (Timeout)
sendto(4, "\377\377\377\0\3SELECT 'xxxxxxxxxxxxxxxxxxx"..., 8192, MSG_DONTWAIT, NULL, 0) = 8192
sendto(4, "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"..., 16769027, MSG_DONTWAIT, NULL, 0) = 219264
sendto(4, "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"..., 16549763, MSG_DONTWAIT, NULL, 0) = 36544
sendto(4, "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"..., 16513219, MSG_DONTWAIT, NULL, 0) = -1 EAGAIN (Resource temporarily unavailable)
sendto(4, "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"..., 16513219, MSG_DONTWAIT, NULL, 0) = -1 EAGAIN (Resource temporarily unavailable)
sendto(4, "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"..., 16513219, MSG_DONTWAIT, NULL, 0) = -1 EAGAIN (Resource temporarily unavailable)
fcntl(4, F_SETFL, O_RDWR)               = 0
shutdown(4, SHUT_RDWR)                  = 0
close(4)                                = 0

mysqlclient does not build with the development branch of MariaDB Connector/C,
due to a missing 'my_config.h' file.

/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 !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.