Use VIF details dictionary to get physical_network

Modify mlnx_direct plug and unplug to retrieve physical_network
from VIF details dictionary if network meta dictionary does not contain
physical_network. This will serve ML2 case following the guidelines to
populate vif_details dictionary with attributes required for port plugging.

Change-Id: I0d97bc875be2fff18087c78accef3ec81c059c4b
Closes-Bug: 1304872
Merge "Use VIF details dictionary to get physical_network"