Add GCE guide and retool a bit to show the add_host interactions, improvements/upgrades are welcome.

Had to shoot the recently merged nova_group module in the head temporarily as it contained a dict comprehension, which means it can't work on all the platforms
and was also breaking docs builds on CentOS.  Will engage with list about that shortly.

Reorg of the documentation tree such that the sidebar will be more correct.  Some work yet to do on module docs.