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.

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 37b515f8, reversing
changes made to ffeeb901.

Please note that re-merging shall be done by reverting this revert and merging
back rest of bug_778748, as described in:
http://www.kernel.org/pub/software/scm/git/docs/howto/revert-a-faulty-merge.txt

This reverts commit da697e8e.

This commit does not seems required.