Remember previous index-allocator allocations(class Subnet). So on adding n+1th subnet on a VN, only reservations for new subnet hit ZK in request context.
Remember previous index-allocator allocations(class Subnet). So on
adding n+1th subnet on a VN, only reservations for new subnet hit ZK
in request context.

Change-Id: Ifedaf1fe9bd03efe546155645461d9b33dd444bd
Closes-Bug: 1591119

nischalsheth/contrail-controller,nischalsheth/contrail-controller,eonpatapon/contrail-controller,codilime/contrail-controller,nischalsheth/contrail-controller,nischalsheth/contrail-controller,eonpatapon/contrail-controller,codilime/contrail-controller,nischalsheth/contrail-controller,rombie/contrail-controller,codilime/contrail-controller,nischalsheth/contrail-controller,rombie/contrail-controller,rombie/contrail-controller,eonpatapon/contrail-controller,rombie/contrail-controller,eonpatapon/contrail-controller,eonpatapon/contrail-controller,nischalsheth/contrail-controller,eonpatapon/contrail-controller,rombie/contrail-controller,nischalsheth/contrail-controller,eonpatapon/contrail-controller,rombie/contrail-controller,codilime/contrail-controller,rombie/contrail-controller,codilime/contrail-controller,codilime/contrail-controller