Multiple fixes in neutronvpn code

Multiple fixes in neutronvpn code

Commit includes:

1> Fixing NPE in NeutronVpnUtils when no network-type is provided from
northbound

2> NVPN: neutron northbound support for gateway mac for external
networks

3> Use vif_type to determine portname prefix

4> NeutronVPN: Handling fixed-floating IP dissociation for floatingIP
delete

5> NeutronVPN additional -ve validations via ECM for network/router
assc/dissc

  a. Added validations for the following:

  a1) Associate network(s) to vpn:
    i) network already assc to another VPN
    ii) network's subnet added to some router as interface

  a2) Dissociate network(s) from vpn:
    i) network's subnet added to some router as interface

  a3) Associate router to vpn:
    i) router assc to another vpn
    ii) network corresponding to router's subnet interface already assc to VPN

  a4) Dissociate router from vpn:
    i) router assc to another vpn

  b. Updating VPNMaps once assc/dissc are done in assc/dissc network(s) from
     VPN

6> NeutronVPN: adding yang model and notifications for router-dpn
map

  a. Added a container to maintain mapping between neutron router and
  DPN(s) on which vpn-interfaces for router are present.

  b. Added notifications for router associated/dissociated to/from vpn

Change-Id: I30931b4ce6013a4bdc0d58ccc86fa8c83d0cdff6
Signed-off-by: Suraj Ranjan <suraj.ranjan@ericsson.com>