[docs] Add scenario customisation information to AIO quick start guide
[docs] Add scenario customisation information to AIO quick start guide

Currently there's no helping information for anyone who wants to
adjust the AIO to add more services. It's also missing information
about how to use scenarios. This resolves that.

Change-Id: I83c3ea2f5229b3af4783e350b7ca156143e5580b

BjoernT/os-ansible-deployment,yanyao/openstack-ansible,cfarquhar/openstack-ansible,robb-romans/openstack-ansible,os-cloud/os-ansible-deployment,openstack/openstack-ansible,BjoernT/os-ansible-deployment,yanyao/openstack-ansible,stackforge/os-ansible-deployment,BjoernT/os-ansible-deployment,BjoernT/openstack-ansible,cloudnull/os-ansible-deployment,BjoernT/openstack-ansible,openstack/openstack-ansible,bunchc/openstack-ansible,cfarquhar/openstack-ansible,os-cloud/os-ansible-deployment,robb-romans/openstack-ansible,bunchc/openstack-ansible,stackforge/os-ansible-deployment,cloudnull/os-ansible-deployment