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.  

Note that the password mappings here aren't actually used by the plugin anymore, so they can be mapped to anything (they just have to be mapped to something in order to keep the plugin manifest happy).