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.

Revert "Revert "Merge branch 'bug_778748'""

This reverts commit 57e5a60d.

In order to have as canonical as possible paths, chomp ../ from filenames and
recalculate base.

Also remove not needed imports.

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