Optimize 'floating_ip_bulk_create' function

The existing floating_ip_bulk_create() DB API method used a for loop and
a session flush operation for each IP created. This patch consolidates
those many operations into a single INSERT statement, which will
perform better than multiple round-trips to the SQL database. We further
optimize the function by making a return of the changed rows optional.

Change-Id: I5cf075f38696110d2890cfa1ea488ca459d791a3
Merge "Optimize 'floating_ip_bulk_create' function"