"test-cloud-meta-mock.py" needs to be mocked, so it only replies
to stuff that we tell it to. Except for the API token, that is
pushed by the client.
We need to be able to tell the mock whether it supports that API
or not. By default it does, but by setting "/.nmtest/providers"
we can limit that.
The DEFAULT_RESOURCE are now grouped by provider. If a path is not
explicitly mocked, we may fallback to the DEFAULT_RESOURCE, but only if
the respective "/.nmtest/providers" is enabled and if /.nmtest/allow-default"
indicates so (which they do by default).
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