obs/slapos: fix buildout version for bootstrapping

by default, the bootstrap script will install newest buildout, but
starting with buildout 3.0.0, there is a problem of wheel like this:

ez_setup.py is deprecated and when using it setuptools will be pinned to 33.1.1 since it's the last version that supports setuptools self upgrade/installation, check https://github.com/pypa/setuptools/issues/581 for more info; use pip to install setuptools
Downloading https://pypi.io/packages/source/s/setuptools/setuptools-41.6.0.zip
Extracting in /tmp/tmpyglw2hza
Now working in /tmp/tmpyglw2hza/setuptools-41.6.0
Building a Setuptools egg in /root/slapos.package/obs/slapos/sn_1.8.3+1.0.290+1/slapos/build/opt/slapos/eggs
warning: no files found matching '*.py' under directory 'tests'
warning: no previously-included files found matching 'pyproject.toml'
/root/slapos.package/obs/slapos/sn_1.8.3+1.0.290+1/slapos/build/opt/slapos/eggs/setuptools-41.6.0-py3.9.egg
Traceback (most recent call last):
  File "/root/slapos.package/obs/slapos/sn_1.8.3+1.0.290+1/slapos/build/opt/slapos/bootstrap.py", line 199, in <module>
    ws.require(requirement)
  File "/root/slapos.package/obs/slapos/sn_1.8.3+1.0.290+1/slapos/build/opt/slapos/eggs/setuptools-41.6.0-py3.9.egg/pkg_resources/__init__.py", line 900, in require
  File "/root/slapos.package/obs/slapos/sn_1.8.3+1.0.290+1/slapos/build/opt/slapos/eggs/setuptools-41.6.0-py3.9.egg/pkg_resources/__init__.py", line 786, in resolve
pkg_resources.DistributionNotFound: The 'wheel' distribution was not found and is required by zc.buildout
Impossible to build SlapOS, exiting.
make: *** [Makefile:9: download_cache] Error 1

obs/slapos/prepare_download_cache.sh

@@ -33,7 +33,8 @@ sed  "s/\%RECIPE_VERSION\%/$RECIPE_VERSION/g;s|\%PATCHES_DIRECTORY\%|$PATCHES_DI
 wget https://lab.nexedi.com/nexedi/slapos.buildout/raw/master/bootstrap/bootstrap.py
 python3 -S bootstrap.py \
   --setuptools-version 41.6.0 \
-  --setuptools-to-dir eggs
+  --setuptools-to-dir eggs \
+  --buildout-version 2.13.8
 
 # 2) get newest version of zc.buildout and setuptools
 #    note that we can't directly do setuptools + zc.buildout +