Neutron L3 may implement a variety of extensions: router, external-net,
dvr, ext-gw-mode, extraroute, l3-ha, etc. The public network uuid is
only going to be made available if and only if the external-net extension
is available, because that's the one that provides Floating IP support.
Rather than making Tempest aware of q-l3 service (when q-* services
are supposed to be legacy), it is better to tune this configuration
based on the extension availability. This decouples Tempest from
Neutron setup internals.
Change-Id: I4889fc3d21bd221785b507995f1b3da0e8f52b46
Related-bug: 1582119
53f59d802f