fixes #6617 - create join table between System and Host for content_view and lifecycle environment

To allow a host to be created before a system (content host) is created and vice versa and to prevent the data from being stored in 2 places, both systems and hosts.

Currently, this information is only retrievable by parsing the puppet environment name connected to the content view / life-cycle environment

I feel like the convention for joins is typically just Model1Model2 - any reason to break that?