CloudSigma provider - constraints (for review)

This pull request is part of juju provider for CloudSigma contaning the following files:

provider/cloudsigma/constraints.go
provider/cloudsigma/constraints_test.go

See the incremental diff here - Altoros#10

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.
I'm sure I mentioned this in the previous PR -- this might be a tolerable first-cut get-it-working hack, but simplestreams integration is required. This should be honouring the `image-metadata-url` config setting; until there's official image data published for cloudsigma, you can use `juju metadata generate-image` to build a simplestreams file including this image id in an appropriate format which you can then host somewhere and point at in your config.