Adds the NeutronDhcpAgentsPerNetwork parameter

Adds the NeutronDhcpAgentsPerNetwork parameter

Currently for both puppet and image-elements based deploys we set
the dhcp_agents_per_network in neutron.conf to 2 and there is no
control over that number (in the hieradata for the former and the
image element for the latter). This change adds the
NeutronDhcpAgentsPerNetwork parameter and also changes the default
to 3 when not explicitly set.

In the puppet case propagate this parameter in the hieradata for
the neutron class and in the non-puppet case expose a new item in
the neutron config to be consumed by the neutron image element
(that change will point here)

Change-Id: Id97c7796db7231b636f2001e28412452cf89562b