lib, zebra: move vrf netns commands from lib to zebra

lib, zebra: move vrf netns commands from lib to zebra

"[no] netns NAME" commands are part of the lib, but they are actually
zebra-only:
- they are using vrf_netns_handler_create and its description clearly
  says that it "should be called from zebra only"
- vtysh sends these commands only to zebra
- only zebra outputs the netns related config
- zebra notifies other daemons about netns attachment

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