Commit 7dbcfae introduced a subnetpool needed for the
auto-configured-topology Neutron extension. However, it allows
only up to 4 tenants, then you'd hit an error about no more
available IPs. This patch changes the size of the subnetpool
and the subnets it creates.
Change-Id: I4f43bebc52fb20e39853a1632fe31506958f5071