Create 'neutron-router' image element

To enable Neutron DVR support, this change introduces the 'neutron-router'
image element, which mirrors the current 'neutron-network-node' element
except it does not have a dependency on the neutron-dhcp-agent element
as that element should not be included on the compute nodes in the
Neutron Distributed Virtual Routing model.

Change-Id: I487ece637b75f8ef3ce0390f2da093b9f123a8d9
blueprint: support-neutron-dvr
Merge "Create 'neutron-router' image element"