Bug7380:CSIT FIP ping is getting failed for Ext Flat/VLAN Network

Bug7380:CSIT FIP ping is getting failed for Ext Flat/VLAN Network

Problem Description:
=======================
FIP Ping TC is getting failed for external FLAT/VLAN network on single
node setup.

For Single node setup VXLAN TEP is not at all required for external
network with provider type as FLAT/VLAN.
Current Implementation on FIP handling, it is
expected nextHop IP should not be null before programming FIP local flow
entry L3_FIB_TABLE (21) -> PDNAT_TABLE (25). This validation is not
required for external provider type as FLAT/VLAN networks.

Solution:
==========
NextHopIP validation is already existing in NatUtil.addPrefixToBGP()
method before FIP advertising to BGP. Hence removing unnecessary
validation from VpnFloatingIpHandler.
This will resolve FIP local flow entry L3_FIB_TABLE (21) -> PDNAT_TABLE
(25) installation on corresponding DPN for external FLAT/VLAN network on
Single Node setup (Without VXLAN TEP Configured)

Change-Id: I6653c78733e037310827ffbcd4498097b19a2abf
Signed-off-by: karthikeyan <karthikeyan.k@altencalsoftlabs.com>