/reviewed-on !610

Caddy and ATS shall have same timeouts for the requests.

It defaults to 600s, which is good reasonable chosen before.

ATS by default tries 4 times (1 non configurable, 3 retries) a given query to
the backend. Despite of "connection" in the name, ATS waits for first byte
from the backend, and it means that backend might already processing the
request, just no bytes are sent. In order to avoid this allow ATS to try only
once.

unlike what I assumed, env argument of ConfigParser.get does not work as
dict.get, it has priority.

This broke the case of SLAPOS-EGG-TEST

Rewrite this using ConfigParser.has_option

  The Remote User is managed on the backend apache which will unset it anyway.

As shown by https://github.com/zopefoundation/Zope/pull/655 and
https://github.com/Pylons/waitress/commit/6d4dab6bed88917b973066a6d5222917661802b7
backends usually don't accept headers with underscores.

SSL_CLIENT_SERIAL is removed because it's unused.

This fixes previous commit.

If backend does not send Content-Type the frontend is responsible of adding
correct one.

Not use erp5-doc as a submodule anymore

/reviewed-on !613

Usually when we make change to SlapOS profiles, we only check the result of SR test and sometimes problems in the json schema are introduced and we only notice this when we check the result of `SLAPOS-EGG-TEST`.

This MR is about:
  - also running `slapos.cookbook` tests as part of `SLAPOS-SR-TEST`. This test in fact contain both "egg tests" for some recipes in nexedi/slapos repository and "lint test" for the formatting of json schemas for parameters (because this nexedi/slapos repository contain both some recipes and profiles).
  - fixing current problems with json schemas on master branch.

/reviewed-on !612

according to rfc4329, text/javascript is deprecated in favor of
application/javascript, so make sure we also compress the "official"
mime type for javascripts.

/reviewed-on !609

Now that we also run slapos.cookbook tests from SLAPOS-SR-TEST we cannot
simply examine buildout.cfg, because unlike SLAPOS-EGG-TEST this
repository is now checked out in parts of the software folder, not the
instance folder.
So that trick of looking up the egg directories in buildout does not
work, because this is a software buildout.cfg ... instead, we assume
that the standard eggs and develop-eggs directories are used.