Don't crash when adding duplicate gre allocation

Don't crash when adding duplicate gre allocation

This patch catches DBDuplicateError when initializing ML2 GRE type
driver and allocation already exists in DB. Because current allocations
are queried and then only those that doesn't exist in database are
added, DBDuplicateError should never occur.

But the race can happen when running multiple neutron-servers and one
of servers adds allocations between allocations are queried and added.

Change-Id: I427b7020d61b0d2c06292ff2804ba1f4483696c6
Closes-bug: 1417560