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.

This test does seem to indicate that you're filtering appropriately for AllInstances.

However, layering-wise I would have thought that code would exist at the juju Provider level, not in the "wrapper of Cloud Sigma's API" level.
Certainly it feels like the knowledge of what environment names are, and how that is represented on instances is very Juju specific, and not a generic knowledge for the gosigma wrapper. (It may be that we supply an easy way to filter the instance list, and then Juju just fills in the details to that filter.)

Also, I would hope that we would filter by environment-uuid and not environment-name. We haven't been great about this in some providers, but since this is new code without backwards compatibility issues, I hope we can do better here. (It may take a bit of thinking for how to handle the bootstrap instance.)