Remove unused db.api.floating_ip_set_auto_assigned

Change-Id: I5a2be6dd733a4bc092e4658d8010cc3a5eb0d6a3
Catch NotImplemented nova exceptions in API extension

Catch currently uncaught NotImplemented nova exceptions
in floating_ip_dns and explicitly convert to HTTPNotImplemented
errors. This avoids tracebacks in the log files.

Change-Id: Idc8d9437920ab0ffcd6cb49db73dae63bcabab0e
Merge "Catch NotImplemented nova exceptions in API extension"
Merge "Remove unused db.api.floating_ip_set_auto_assigned"