Default to /24 prefix for floating IP range with neutron

When running Tempest parallel tests with neutron, several long-running
tests might create routers and floating IPs, which will result in
IP allocations over the public network.

Increasing the public network size should ensure tests do not fail
due to IP address shortage; this patch also updates the public network
gateway IP address.

Related-Bug: 1253966

Change-Id: Ie075b3c4d14a07b06c42fd29b09770dd1972aa45
Merge "Default to /24 prefix for floating IP range with neutron"