fabrics: first read config before topology scanning
The topology scanning will create nodes in the internal libnvme tree. As
we rely on the order of the nodes in the tree, we need to read the
config before we scan. This makes sure the first node is the one we have
defined in the config.json. This is important because the first node
provides default hosnqn configuration in absents of /etc/nvme/hostnqn.