nixos/ceph: run unprivileged, use StateDirectory and tmpfiles, don't pass extraServiceConfig

Don't pass user and group to ceph, and rely on it to drop ceps, but let
systemd handle running it as the appropriate user.

This also inlines the extraServiceConfig into the makeService function,
as we have conditionals depending on daemonType there anyways.

Use StateDirectory to create directories in
/var/lib/ceph/${daemonType}/${clusterName}-${daemonId}.

There previously was a condition on daemonType being one of mds,mon,rgw
or mgr. We only instantiate makeServices with these types, and "osd" was
special.
In the osd case, test examples suggest it'd be in something like
/var/lib/ceph/osd/ceph-${cfg.osd0.name} - so it's not special at all,
but exactly like the pattern for the others.

During initialization, we also need these folders, before the unit is
started up. Move the mkdir -p commands in the vm tests to the line
immediately before they're required.