zebra: fix vni configuration in default vrf

zebra: fix vni configuration in default vrf

VNI configuration is done without NB layer in default VRF. It leads to
the following problems:

```
vtysh -c "conf" -c "vni 1"
vtysh -c "conf" -c "vrf default" -c "no vni"
```
Second command does nothing, because the NB node is not created by the
first command.

```
vtysh -c "conf" -c "vrf default" -c "vni 1"
vtysh -c "conf" -c "no vni 1"
```
Second command doesn't delete the NB node created by the first command.

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