Expose Nexus plugin settings

This patch introduces composition layer changes that show users
how to configure the Cisco Nexus plugin in h.2 using the
coe::nexus class.

Partial-Bug: #1276168

Note to reviewers: I've opted to do this as a patch against the Cisco version of puppet_openstack_builder for the time being because the interface we're using here is temporary and will change in i.0 when we move most of this logic into puppet-neutron.  At that time we may opt to make additional hierarchy changes that simplify the config further.