Inside of slapos.cfg you can define the section called networkcache,
which should contain all the required configuration to upload files
to shacache and shadir.

Options:
  - upload-cache-url: url to upload content to shacache
  - upload-dir-url: url to upload content to shacache
  - signature-private-key-file: private key file used to sign the
    information which may be sent to shadir

Note: the option signature-private-key-file, may also be used as
parameter of slapgrid command.
   (i.e. --signature_private_key_file=/path/to/private.pem)

And of course, following the previous commit, we need to propagate these
parameters to Software class.