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

@isratrade if you ask me, all the join tables are confusing in their own right, but for me I'd rather follow convention. If a majority would prefer to name it this then I am fine with it.