Fix default for neutron.dhcp_agents_per_network

In the original PR, invalid configuration was specified for
neutron.dhcp_agents_per_network.

(cherry picked from commit 177ba950e31c4b0fa3e1c3ab98609d26cbbd392e)
Merge pull request #153 from claco/pr-152-backport

Fix default for neutron.dhcp_agents_per_network