Add GCE guide and retool a bit to show the add_host interactions,…

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.

CHANGELOG.md

@@ -17,7 +17,6 @@ New Modules:
 * system: locale_gen
 * cloud: digital_ocean_domain
 * cloud: digital_ocean_sshkey 
-* cloud: nova_group (security groups)
 * cloud: nova_fip (floating IPs)
 * cloud: rax_identity
 * cloud: ec2_asg (configure autoscaling groups)

docsite/rst/guides.rst

@@ -8,6 +8,7 @@ This section is new and evolving.  The idea here is explore particular use cases
 
    guide_aws
    guide_rax
+   guide_gce
    guide_vagrant
    guide_rolling_upgrade