"nm-cloud-setup" can by configured via environment variables. Mark all the
names of such variables with NMCS_ENV_VARIABLE() macro. This allows to grep
for them.
"nm-cloud-setup" is supposed to work without configuration.
However, it (obviously) fetches data from the network you are connected to (which
might be untrusted or controlled by somebody malicious). The tool cannot
protect you against that, also because the meta data services uses HTTP and not
HTTPS. It means, you should run the tool only when it's suitable for your
environment, that is: in the right cloud.
Usually, the user/admin/distributor would know for which cloud the enable the tool.
It's also wasteful to repeatedly probe for the unavailable cloud.
So, instead disable all providers by default and require to opt-in by setting an
environment variable.
This can be conveniently done via `systemctl edit nm-cloud-provider.service` to
set Environment=. Of course, a image can also pre-deploy such am override file.
We don't want that when the user installs the package, that the
dispatcher script automatically executes the tool. Instead, the user
should use `systemctl enable/disable` to control whether the service
is active (of via the timer).
Hence, let the dispatcher script check whether the service is enabled.
That leads to a different problem, that we need to make it possible for
"nm-cloud-setup.service" to be enabled in the first place. As such, add
a [Install] section and let it be wanted by NetworkManager.service. The
problem with this is that now the tool will run very early, just after
NetworkManager started. At that point, it might not yet have setup
networking. But that should be acceptable, after all, the tool either
fails to fetch meta data that early, or it succeeds. Very likely, it
will by aborted by dispatcher's restart command.
