bgpd: Incorrect auto-RT formed when L3VNI is not configured

bgpd: Incorrect auto-RT formed when L3VNI is not configured

We use ASN:VNI format to calculate auto RT for L3VNI.
When L3VNI is not configured, if we delete the configured RT, incorrect auto-RT
value is generated as VRF VNI is 0.

Fix:
Do not configure auto-RT if L3VNI is not configured.

Trigger:
1. Delete L3VNI
2. Delete configured RT.

Before fix:

dev# sh bgp vrf vrf-blue vni
BGP VRF: vrf-blue
  Local-Ip: 10.100.0.1
  L3-VNI: 0
  Rmac: 00:00:00:00:00:00
  VNI Filter: none
  L2-VNI List:

  Export-RTs:
  RT:101:0
  Import-RTs:
  RT:101:0
  RD: 10.100.0.1:2

After fix:

dev# sh bgp vrf vrf-blue vni
BGP VRF: vrf-blue
  Local-Ip: 10.100.0.1
  L3-VNI: 0
  Rmac: 00:00:00:00:00:00
  VNI Filter: none
  L2-VNI List:

  Export-RTs:

  Import-RTs:

  RD: 10.100.0.1:2

Signed-off-by: Ameya Dharkar <adharkar@vmware.com>