VxlanMgr changes for EVPN VXLAN

1. Add support for EVPN NVO config table.
2. Changes to VxlanTunnel create handler to support KLISH and openconfig.
   The VXLAN_TUNNEL table in the config db gets populated as follows.
   NULL,NULL - interface vxlan create
   NULL,NULL,src_ip,<a.b.c.d> - when SIP is set.
   NULL,NULL - when SIP is unset.
   src_ip,<a.b.c.d> - This format is when click is used.
3. VxlanTunnel and TunnelMap handlers are enhanced to also cache the data so that
   this can be used during the deletion.
4. ARP suppression changes
   Create state db for vxlanmgr and vlanmgr to sync the tunnel association to vlan
5. Changes to support warm reboot.

HLD Location : https://github.com/Azure/SONiC/pull/437/commits

Signed-off-by: Rajesh Sankaran <[email protected]>