Allow port mac_address to be modified

Allow port mac_address to be modified

With ironic servers, a NIC can fail and be replaced with one that has a
different mac.  The corresponding neutron port needs to be updated with
the new mac address so the NIC can be guaranteed to retain the same IP
address.

This change enables this feature in the ml2 plugin.  There may need to
be changes to other plugins to disable or complete the implementation of
this feature.

Closes-Bug: #1341268
Partially-implements: blueprint allow-mac-to-be-updated
APIImpact: Ports
DocImpact: minor port update api change
Change-Id: I1864c0882cda7eddc9ced519ed3f96c91b2b63f3