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

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.

This is actually quick region validation, I agree it looks confusing. Probably the clean way would be to add gosigma function to check if region code is correct (now 'zrh' or 'lvs') - does that make sense?