Raise proper exception in case duplicate ipv6 address is allocated

In case neutron tries to generate ipv6 address for slaac subnet,
it doesn't check that generated ip is unique and throws DB exception
that results in 500 HTTP error.

The fix throws proper IpAddressInUse exception.

Closes-Bug: #1358731
Change-Id: I66aed81b8d31d1eff6c87a77c4c4a813179ba458
Merge "Raise proper exception in case duplicate ipv6 address is allocated"