When using slapos.reboostrap to re-launch buildout with the version
of Python that was just installed, zc.buildout and its dependencies
are reinstalled for the new Python with the same egg versions. Thus
SRs which install Python 2 need to use Python-2-compatible versions
of setuptools, pip and wheel all along.

For this stack/slapos-py2.cfg is introduced.

It is also used to move all

```
[openssl]
<= openssl-1.1
```

from individual Python2 SRs to a common place.

Change python2 softwares to use python2 explicitly and python3
softwares to use the default, but leave a few softwares (html5as,
html5as-base, fluent-bit, packer, seleniumrunner) using the default
python, as they were using python2 not because they only support python2
but because they don't have strong dependency to the python version
being used.

Use slapos.recipe.template instead of slapos.recipe.template if possible

Harmonize template keys :
rendered -> output
template -> url
template = inline: -> inline =

Delete "mode" key

See merge request nexedi/slapos!1151

Most of required eggs are anyway available on pypi, which is much more
reliable than all scattered sources. Idea by Leonardo.