This reworks the cloud metadata mock server in a significant way.
Most importantly this makes it possible for the client to add and
modify the resources for later retrieval using the PUT method.
This allows the test to create the fixture for itself.
The default set of resources is still provided, so that the too remains
useful as a development aid. If that is not desirable, the --empty
parameter might be passed to cause the server to start with no
resources.
"preexec_fn" is not great, because it is not generally safe in multi
threaded code (and we don't know whether the test didn't start other
threads already, like a GDBus worker thread). Well, it probably is safe,
if you only call async signal safe code, but it's not clear what os.dup2()
does under the hood.
Just avoid that. We can pass on the FD directly.
