This reverts commit b6730a8b.

Well, according to this commit at slapos.core.git:
  - 017a02ef202b69dff03e85de18293bc30ac6b545

This parameter is always under networkcache section to keep the
configuration consistenty.

Well, according to the commit d8dca883a54ee87aafed3a82921594e5b508b388
at slapos.core.git, this parameter is always under networkcache section
to keep the configuration consistenty.

Renamed signature_private_file to signature_private_key_file.
And also signature_public_file to signature_certificate_file.

Both files must be generated by opensssl.
- signature_public_file: used to verify if the content is trustable by
  libnetworkcache
- signature_private_file: used to generate the signature before uploading
  the content to shadir by libnetworkcache.

As networkcache is really optional, silence it in case if not enabled nor not
availble.

Shacache extension shall not influence buildout run in case of server
malfunction.

upload part is optional.

URLs are important to being read, not files.

For now it is done by using external networkcache server. Integration of
simulator is WIP.

Create additional parts in profile.

When you use 'for line in file', it uses the next method under the hood.
And such method can load very big pieces of the binary file into the RAM
memory.

The shutil method will copy the file using smalls chunks, so the
RAM memory is gonna be safe.

So it needs to write the file content in the correct path which must
be used by buildout.

Also remove not needed imports.

If slapos.libnetworkache is not available, setup environment in no-op way.

Keep same ordering and simplify conditions.

If it is possible to use slapos.libnetworkcache check if parameter
networkcache-section exists. If so, try to find section named by this parameter.

Such section has two required parameters: download-cache-url and
download-dir-url. upload-cache-url and upload-dir-url are optional parameters.

Minimalistic profile for networkcache control looks like:

  [buildout]
  parts =

  networkcache-section = readcache

  [readcache]
  download-cache-url = http:///
  download-dir-url = http:///
  #upload-cache-url = http:///
  #upload-dir-url = http:///