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:///

This reverts commit da697e8e.

This commit does not seems required.