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.
Ah, sorry, I see. But I think the earlier questions come back into play... does this really mean "bootstrap", or "state-server"? If the former, why don't subsequent state servers get storage? If the latter, why's it called bootstrap? And, in general, why are we setting a drive size of 0 for other machines? Presumably they don't actually come with *no* storage when that happens -- but regardless, why are we trying to distinguish at this level, and what's the actual impact?