Renamed signature_public_file to signature_certificate_file, because the
name is cleaner once the content of the file is a X509 certificate
file. It is also known as Public Key Certificate.

Renamed signature_private_file to signature_private_key_file, because
the is is cleaner once the content of the file is a private RSA key.

Actually, the slapos.libnetworkcache has new dependeincies because
M2Cryto is required to handle the signatures inside NetworkcacheClient class
and the argparse is required by signature.py script.

Now it is possible to create the signature files easily using the
command generate-signature-key with the correct arguments.

The command generate-signature-key works without any argument as well.

To check the arguments you must run with the parameter --help/-h.

It'll generate two file, one is a private key and the other is a
X509 certificate file.

Those files must be used by networkcache to play with signature.

The private key is required to sign the message before uploading it to
shadir. And the certification file is required to verify the signature.

M2Crypto is required to create the signature string and verify if it is
valid or not.

The signature private file is not required to verify the signature it is
only used to create the signature string.

The signature verification requires only the public file...

We should not send the signature in the property dict. Otherwise,
the validation of json schema will fail.

Also fix formatting a bit.

It should return the file descriptor instead of file content as string.

This implementation does not consume RAM Memory, so it is safer to
download big files.

When you call file_descriptor.close, it will remove the file from tmp
folder automatically.

There is no need to read and store downloaded file. Returning the location is
good enough.

Rest of file requires hexdigested file.

There is no need to read whole file into memory on library level. So calculate
sum in chunks and give request method file descriptor.

Indeed, some day documentation shall be written, as soon as API will be defined.

HTTPConnection accept port as none, there is no need to play with it.