Allow updating VM port's mac_address

To allow nics on ironic servers to be replaced while retaining the same
IP address, we allow unbound or failed VM ports to have their
mac_address updated.

Change-Id: I83808f7a7f09a7d63a7030843eff74fc364cddcc
Implements: blueprint allow-mac-to-be-updated
Merge "Allow updating VM port's mac_address"