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.
...and I'm not sure they should exist yet anyway. I thought this CL was just config? (and constraints -- not clear what they have to do with config, but at least the diff is under 1000 lines now...)