Catch duplicate errors scheduling SNAT service

Catch DBDuplicateEntry errors when scheduling SNAT to
a service node to prevent harmless tracebacks in the log.
These can occur if scheduling occurs concurrently,
so they are safe to ignore.

Closes-Bug: #1381958
Change-Id: If242b04b372609f640f3ce88f4245c17a45bf69d
Merge "Catch duplicate errors scheduling SNAT service"