Update the ml2 flat_networks configuration to set '*' as the
default because that has been effectively the default behavior due
to a bug with the empty list.
Also, fix the empty list configuration for disabling flat networks.
DocImpact
UpgradeImpact
Change-Id: I13eb22afd7c0d011154b58a1bd218fd4fa82a8d9
Closes-bug: #1437745
Co-Authored-By: Martin Hickey <martin.hickey@ie.ibm.com>