fabrics: Consider config from file when adding new controller

fabrics: Consider config from file when adding new controller

nvme_read_config() function is attaching the configuration
to tree. But when we create a new controller via nvme_create_ctrl()
and then call nvmf_add_ctrl() we ignore this previously read
in configuration.

Hence lookup if there exist a controller/config and merge into
the fabrics config.

Note, the order of the merge is important. For example we want
the config from the command line to have higher priority
than the one from the config file.

Signed-off-by: Daniel Wagner <dwagner@suse.de>