Securing targets in multi-provider context

[carns]

In GitLab by @mdorier on Jan 15, 2018, 09:53

Right now it is possible to send a bake_write, bake_read, or bake_persist request to a provider that is not actually in charge of the pool underlying the given region id. We should add some checks to make sure the requests are sent to the appropriate provider, and add test cases that should fail if one tries to access a provider's pool through another provider.