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 line seems a little confusing to me, especially since 
https://github.com/Altoros/gosigma/blob/master/client.go#L50
seems to be repeating it.
I believe the goal is to allow you to specify a short or a long region name, but in that case shouldn't ResolveEndpoint just return a full URL if it is supplied?
Certainly the fact that gosigma itself calls ResolveEndpoint makes it a bit muddled as to what the value is expected to be.
Is it just that we want to get the best value here we can for logging purposes? As it would appear that if you got an error here, you're going to just get that error again 10 lines down in gosimga.NewClient().