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.

You can still do this with external tests, you just need to have an:
export_test.go:
  var NewClient = &newClient
and then you can
s.PatchValue(cloudsigma.NewClient, ...)