fsm: Add ni_ifworker_is_config_worker() function

This function determines whether given worker has real
device behind or is just a config container.
fsm: ni_fsm_ifworker_by_netdev() should not match by name
fsm: Match ready devices against config workers (bnc#904061)

Sometimes device is renamed just after we create a config
worker for it. Then real worker resides along with config worker.
Every further worker get function returns the real worker, which
does not have any configuration.
Solution here is as follows:
- Refresh all devices present in the system (ready or not) at
  discovery (ni_fsm_refresh()) state
- Create new workers for hotplugging devices only for ready devices
- During refresh assign new device (when ready!!!) to the worker
  matched by name.
  * If that happened, look for real workers having similar ifindex
    and destroy them (we do not need them, as config worker becomes
    real worker now).