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.

This function should check not whether or not config changed, but whether the fields, that were used when we create new client, were changed. If this return true - we should recreate the client.  If we will call configsDifferent - that would be not clear. Now this function is moved to Config and renamed