Cloudsigma provider - config and constraints

Juju provider for CloudSigma.

Provider for CloudSigma (https://www.cloudsigma.com) uses CloudSigma API v2.0 (https://cloudsigma-docs.readthedocs.org/en/2.10/).

Minimal client for communicating with the CloudSigma Web API from Go program was implemented as separate library https://www.github.com/Altoros/gosigma. This library is published under AGPLv3 license.
I don't think this is right. I'm assuming that a driveTemplate is basically an image? If so, this should be coming in from simplestreams data, so we don't need to change the provider every time we release a new OS. Basically any direct OS dependency in the provider is a layering violation.