Optimize 'fixed_ip_bulk_create' function

1) All single 'insert' operations were changed by
single 'multiline insert' operation

2) Explicit flush() was removed

3) Using code from oslo.db provides
getting error value from DBDuplicateEntry exception

Change-Id: Ib3d99b950bcf5e97ac5d6099d5ccfbe98906b339
Merge "Optimize 'fixed_ip_bulk_create' function"