Handle DBDuplicateEntry on Ports with same address

Handle the DBDuplicateEntry exception when creating/updating ports with
the same address and raise MACAlreadyExists to give user a better feedback
about the problem.

Change-Id: I87feb27776bb2fee2a4a6b28f9468e235063f5f7
Closes-Bug: #1257673
Merge "Handle DBDuplicateEntry on Ports with same address"