Support system scope metadata schemas

[machristie]

Some metadata records will adhere to schemas that are the same across all tenants. For example, Airavata will have a metadata schema for experiment instances and that schema would apply to all tenants that have Airavata experiments. So it would be useful to not have to redefine global schemas again for each tenant.

On the query side of things, I think the API will support a query that looks like this:

select * from system.experiment where ...

where experiment is a system scoped metadata schema.