Otherwise, the logging output is confusing as it's not clear what happened about
the provider detection.

Also, don't return from _provider_detect() unless all pending callbacks
returned. It is ugly to leave callbacks pending, because they will be
later dispatched when the caller iterates the GMainContext further.
Instead, cancel pending operations bug keep running until we processed
all cancellation callbacks.

It's the same code, four times. And once it was messed up.
Move the definition of the function to a macro.

In production systems, the associate an interface by their permanate MAC address.
For testing and CI, we may want to hook that. That allows for example to run
nm-cloud-setup on a veth interface, which doesn't have a permanent MAC address.

This is only for testing.

Previously, there was NMCS_ENV_VARIABLE() macro. That macro did nothing,
it merely acted as something to grep for, when searching the source for
which environment variables nm-cloud-setup honors. That is an
interesting thing to know, because nm-cloud-setup is configured via
environment variables.

Change that. Instead add a define for each environment variable. You can
now instead grep for "NMCS_ENV_" to find them all.

Fixes: f3404435a9fb ('cloud-setup: configure secondary ip in Aliyun cloud')

"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 not 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.

Choose a MAC address that is easily recognizable. And where you can
see which is mac1 and mac2.