For now this entry is not used but it will be used in future versions of
slapcache. So make sure that from now on, the entry exists.

textwrap.indent doesn't exist in python2.

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.

See merge request !4

See merge request !5

See merge request !6

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

* revive tests (only what was already there)
* `upload` command supports python3
* better log message
* temporary files are removed

See merge request !3

/reviewed-on !2

  ca file is required for work with Python>=2.7.9