Create l3vpn & asso with ext-nw failed

Create l3vpn & asso with ext-nw failed

Issue:
=======
Creation of SNAT use case via REST API fails creating vpn
(Creation and Association to VPN in Single REST API Call)

Using Jumbo RPC Call to create the internet BGP-VPN instance(l3vpn)
and at the same request call associate the external GRE provider network
also in a single REST API call is FAILED due to absence of vpn-instance
OPERATIONAL data store.

Solution:
==========
NeutronVPN YANG model has redesigned properly to work internet VPN
use case for both V4 and V6 networks.

"vpn-instance-op-data" should be created/updated/removed
by VPN Module only. Since it is VPN drivern OPERATIONAL data store.

Have modified the NeutronVPN design to just remove the
"vpn-instance-op-data" from NeutronVPN module to VPN Module.

"vpn-instance-op-data" is populated based on the CONFIG vpn-instance
data.

Signed-off-by: Karthikeyan Krishnan <karthikeyangceb007@gmail.com>
Change-Id: I6ff3e54c3302eb0ee3c6f238dc27f2567ca5f72a