Use db constraint to ensure mac address uniqueness

Use db constraint to ensure mac address uniqueness

Currently port mac address uniqueness per network is checked before Port
db object create but without locking. It implies 2 port create requests
can allocate the same mac address on a network if each request performs
mac address uniqueness check before the other creates the Port db object.

This change replaces the check by a db unique constraint on Port
(network_id, mac_address).

Change-Id: Iad7460356ceb74d963cddf5ec33268d77792f1fe
Closes-Bug: #1194565