Support device sharing between clouds, regions, and cells

[Jim Baker]

Certain devices in deployed OpenStack clouds may be shared between clouds, regions, and/or cells. See http://cdn.pasteraw.com/90abpeqkkw8sa0zxaa1vj3jm03ld5rl

This means that the current modeling in Craton's schema is too rigid, which assumes a single parent in this hierarchy. Instead, we need to support a device being potentially in multiple trees.

This functionality is closest to what was earlier implemented in labels, when labels could themselves have variables; and it uses standard association tables and corresponding SQLAlchemy modeling to implement.

Current REST APIs can remain mostly unchanged, except that request/response bodies where region_id, cell_id, and (newly) cloud_id will now be a list of IDs, not just a single ID.