Juju provider for CloudSigma - environinstance (for review)

This pull request is part of juju provider for CloudSigma contaning the following files:
- provider/cloudsigma/environinstance.go
- provider/cloudsigma/environinstance_test.go

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'm guessing "sigmaEnviron" is a carryover?
At the very least this seems more "Debugf" level rather than Infof (info => shown by default in the log).
If it had more details it might be useful, but just a "start instance was called" doesn't seem like it adds a lot to the log.