WARNING: this fixes a very old bug that download-binary-dir-url was used
in place of download-dir-url. This may break existing instance of
slapcache.

See slapos.libnetworkcache@0fe8148a

slapos.libnetworkcache now needs the signature key to have a
corresponding certificate in signature-certificate-list so we need to
give signature_certificate_list to NetworkCacheClient constructor.

Also remove mentions to signature-certificate-file in example
configuration.

Otherwise user might spend a lot of time debugging problem of slapcache-upload
misbehaving.

This gives user the possibility to check what was uploaded to shacache.

Use same option name as libnetworkcache so that config files are compatible

  ca file is required for work with Python>=2.7.9