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

See the incremental diff here - https://github.com/Altoros/juju-cloudsigma/pull/3

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.

Name was required to be stored inside client, because name was attached in metadata to each new machine, that we create, and later used for filtering. now instead of name id is used (as was requested in comments for our pull request) username and password were used to check, whether configuration is changed, and whether we should recreate gosigma client. Now in cloudsigma_provider_simplestreams branch this code was moved to config.ant those fields were removed.