lib, mgmtd: don't register NB config callbacks in mgmtd

lib, mgmtd: don't register NB config callbacks in mgmtd

mgmtd is supposed to only register CLI callbacks. If configuration
callbacks are registered, they are getting called on startup when mgmtd
reads config files, and they can use infrastructure that is not
initialized on mgmtd, or allocate some memory that is never freed.

Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>