Convert buildout emptiness to Nones.

Certificates to check content are provided inline.

Use signature_certificate_list if strings instead of signature_certificate_file.
Additionaly simplify backward compatbility: just do not play with it.

Use simple download recipe instead of relying on egg downloading.

In addition comment why it does not work.

Instead use provided system level tools to generate keys.

This reverts commit bfe5fb83.

After merging the master branch inside the signature branch of
slapos.libnetworkcache it is required to fix some error messages.

Thanks to 4784a35e, which does buildout
"patching" procedure in test it is not required to provide testnc.

By the way revert "Revert "Revert "Allow to find slapos.libnetworkcache during
test.""" (commit 0fbf2ccf ), as it is not
required anymore.

Whenever special sign ? is found in url analyse it in order to have
correct filename.

In case of http://host/path/filename/? use "filename" as part to
generate filename.

This fuzzy logic allows to generate quite sane filenames.

The changes were made in the previous commit:
 - 75a7c904

We should only create the signature files when it is required, to avoid
to have unecessary failures if someone run this test with old version of
slapos.libnetworkcache.

This reverts commit 01b2ca0a.

The slapos.libnetworkcache == 0.2 does not provide the signature script.

So, it should not be used if such script is not available.

Well, the new version of slapos.libnetworkcache has being developed,
and we need to provided backward compatibility to the old versions.

So, slapos.buildout must be able to instantiate a NetworkcacheClient
object even if an TypeError is raised.

As slapos.libnetworkcache development shall be outside on buildout, do not
create hard dependency.

This reverts commit 4dc2aef8.

For now this is the only working way to test buildout with
slapos.libnetworkcache.

slapos.libnetworkcache development shall be NOT zc.buildout driven.

This reverts commit e3345a7a.

Doing development of zc.buildout parts which shall depend on
slapos.libnetworkcache without ability to develop it locally is nowhere path.

This reverts commit bacd2505.

Doing development without local slapos.libnetworkcache has no sense.

By having newline in interpreter name, z3c.recipe.scripts created
bin/\npync file, and such was returned. This lead to returning
buildout:bin-directory path, which was killing buildout in next run.