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.
Are there also environment variables for these settings?
Most of the providers at least support setting username/region/password in ENV vars, though that is generally done in the lower level library.
If you look at the openstack provider inside of "Validate" they call "cred := identity.CredentialsFromEnv()" which is where the logic lives for what ENV vars to use.