BGPVPN delete is throwing NPE

BGPVPN delete is throwing NPE

Issue Description:
=================
BGPVPN not getting Deleted and remaining stale due to the
exception[0] from CSC

[0]
<h3>Caused by:</h3><pre>java.lang.NullPointerException
at
org.opendaylight.netvirt.neutronvpn.NeutronvpnManager.removeVpn(NeutronvpnManager.java:2191)
at
org.opendaylight.netvirt.neutronvpn.NeutronvpnManager.deleteL3VPN(NeutronvpnManager.java:1443)

Solution:
=========
If BGPVPN instance has already disassociated from
Network(s), later user is trying to delete the BGPVPN
instance using REST API call, we should handle
proper null check to avoid the exceptions from
NeutronVpnManager. Since vpnmaps querying is causing
this Nullpointer Exception

Change-Id: I17037a52862cf851985931eff94a88bcfc951476
Signed-off-by: Karthikeyan Krishnan <karthikeyangceb007@gmail.com>