Juju provider for CloudSigma - instance (for review)

This pull request is part of juju provider for CloudSigma contaning the following files:
- provider/cloudsigma/instance.go
- provider/cloudsigma/instance_test.go

See the incremental diff here - https://github.com/Altoros/juju-cloudsigma/pull/4

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.

We need to make sure that we are fully satisfying the interface.

var _ instance.Instance = (*sigmaInstance)(nil)