northbound: Split out AD-SAL logic from AbstractNeturonNorthbound

northbound: Split out AD-SAL logic from AbstractNeturonNorthbound

This is a follow up patch for
  changeset of 43f88bf737adcb2e5df9e106d8b491de1549c41d
  change id of I3de3f9461047363501f0e3a6428580f93402c65b
  Neutron*Northbound: consolidate crud logic
and
  changeset of aafe31458a9990f50023566aed7f7b296b95067c,
  change id of I2c36c104dbd6c891a36c10419ed28d8060958d29.
  BGPVPN: Added yang, api and transcriber for BGPVPN

bgpvpn which was newly introduced after AD-SAL deprection doesn't implement
AD-SAL. So the class of NeutronBgpvpnsNorthbound requires its super class,
AbstractNeutronNorthbound, not to use AD-SAL.
This patch factors out ad-sal logic from AbstractNeutronNorthbound
into new subclass, AbstractNeutronNorthboundIAware with minor compilation
adjustment.

Change-Id: I9ce990558f178f1571cfac9760dd333b5b40174f
Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>