Make the docs more specific.

Also remove a few whitespaces, improve the bash script and fix a typo.

Clarify which of the options to pick.

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.