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