add a control plane API for mimic to specify a behavior for "what kind of cloud do new tenants have"

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We want to be able to support multiple types of "cloud"; for example, "reflect as accurately as possible the attributes of Rackspace Public Cloud", or, "... Rackspace Private Cloud" or "... DevStack". Right now, Mimic mostly looks like Rackspace public cloud, with various differences, but as things like the image list are made increasingly specifically accurate for testing the cloud control panel, Mimic is coming to reflect public cloud a bit too precisely.

What-cloud-is-this state is mostly represented on a per-tenant basis (for example: images go into a tenant-specific store), so we can model this as a tenant-creation Behavior parameter as a mimic-global control plane; when you create a tenant (matching criteria X, probably based on username), give it this pre-defined cloud. We can then define a plugin for "cloud types", probably starting off mainly with a list of datacenters and a list of images. Later we will need to expand this on a per-plugin basis; each plugin might have its own per-cloud state; also, we will need to adjust which plugins appear in each "cloud", because, for example, public clouds tend to have a lot more services available in the catalog than e.g. DevStack.