zebra: Only create vrf route-node if in the kernel

zebra: Only create vrf route-node if in the kernel

Changed vrf_get to only create a route-node if the kernel has enabled the vrf,
supplying a table-id.  If the quagga configuration contains vrfs that have not
yet been enabled by the kernel, the vrf is created with a vrf_id of VRF_UNKNOWN.
These vrfs are added to the vrf_list so they contain a zvrf and configuration,
but the route-node is not created until the kernel has signalled that the vrf
has been defined and enabled.

Ticket: CM-10481
Signed-off-by: Don Slice
Reviewed-by: Donald Sharp