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.

How are you handling the very specific warning from the comment above? Specifically, how are you tracking what instances are part of *this* environment?
The code I see written here looks like it will return all instances controlled by my user. Which means I would be unable to run 2 environments with the same user credentials.
CloudSigma lets you apply tags to instances, and it seems perfectly appropriate to do something like put "juju-environment: ENVIRON_UUID" on all instances that are part of an environment, and then filter on that here.