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

@ehelms, at first I called it `SystemHost`, but this was confusing to some of the people because of the past discussions between Systems and Hosts. I wrote on irc for suggestions. Some where Contentable, ContentHostAttribute, SystemHostCombination, SystemHostCommon. It doesn't matter to me as this is totally transparent and not exposed to the UI or API.  Open to ideas...