Juju provider for CloudSigma - client (for review)

This pull request is part of juju provider for CloudSigma contaning the following files:
- provider/cloudsigma/client.go
- provider/cloudsigma/client_test.go

See the incremental diff here - https://github.com/Altoros/juju-cloudsigma/pull/1

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 generally considered instance as started when it was created by the cloud and IP address assigned - so it's reachable via network. Sometimes we saw delays up to couple of minutes in instance provisioning by CloudSigma - but current code waiting for server start and IP address assigned proved to be robust. Does this make sense? Are there some additional checks in juju code for successful start of an instance?