Add MacAddressInUseClient exception handling

Nova needs a specific client exception defined for the server's
MacAddressInUse exception when allocating ports so it can handle the
exception properly, otherwise the NeutronClientException with a 409 can
be confused with other errors.

Partial-Bug: #1347778

Change-Id: Ia02dbc8fe32a43adeb229e3b640b9b33ec0dd6c7
Merge "Add MacAddressInUseClient exception handling"