Preserve link local IP allocations for DVR fip ns across restart

Preserve link local IP allocations for DVR fip ns across restart

The L3 agent allocates link local address pairs used in connecting the
routers to the floating ip namespace.  When those allocations are
forgetten by restarting the L3 agent they all get rewired on restart.
This change preserves the allocations using a file in the local file
system.  Storing them in the database would be overkill and would
affect system performance.

Change-Id: I39614c7ea2a7dcc35bf969c90045adc5926ea9df
Closes-Bug: #1348306
Partially-Implements: blueprint neutron-ovs-dvr
Co-Authored-By: Rajeev Grover <rajeev.grover@hp.com>