Bug 5010: NeutronVpn: Internal VPN delete/recreate redesign + related fixes

Bug 5010: NeutronVpn: Internal VPN delete/recreate redesign + related fixes

1. Eliminated deletion and recreation of internal vpn on
association/dissociation with vpn
2. Fixed router deletion - earlier internal vpn wasn't getting deleted
3. Updated getVpnForRouter to handle multiple VPNs after change (1), for
querying for either of internal/external vpn
4. Router deletion not recreating interfaces for internal vpn and clearing
from vpnmaps in multiple steps - cleaned for efficiency purposes
5. Updated to not fetching Neutron Router DS for interfaces once router is
deleted, using DCN input instead
6. Cleaned up yang imports to not use entire package, earlier done due to
conflicting packages

Change-Id: I4aa6568ac6c724cf9f361534e7df9e0807d2b5a9
Signed-off-by: Abhinav Gupta <abhinav.gupta@ericsson.com>