[auth] Use pyramid_multiauth
It makes it easier to finely tune what parts of the default authentication stack
we want to use or not.
It also makes it possible for any cube to add its own policy in addition to the
others.
Related to #4985962
[pyramid\_cubicweb][] is one specific way of integrating [CubicWeb][] with a
[Pyramid][] web application.
### Features
* provides a default route that let a cubicweb instance handle the request.
### Usage
To use, install pyramid\_cubicweb in your python environment,
and then [include][] the package:
config.include('pyramid_cubicweb')
### Configuration
Requires the following [INI setting / environment variable][]:
* `cubicweb.instance` / `CUBICWEB_INSTANCE`
[pyramid\_cubicweb]: https://www.cubicweb.org/project/pyramid-cubicweb
[CubicWeb]: http://www.cubicweb.com/
[Pyramid]: http://pypi.python.org/pypi/pyramid
[include]: http://docs.pylonsproject.org/projects/pyramid/en/latest/api/config.html#pyramid.config.Configurator.include
[INI setting / environment variable]: http://docs.pylonsproject.org/projects/pyramid/en/latest/narr/environment.html#adding-a-custom-setting