[doc] Adjust deploy-guide to install py3
[doc] Adjust deploy-guide to install py3

As we don't need py2 now, we should adjust host preparation guide
to install py3 isntead.
Additionally patch extends explanation regarding lvm usage for lxc storage

Change-Id: Idc9897d634e06df4c1084b3a3876b3a3971a0a6d

openstack/openstack-ansible,openstack/openstack-ansible,stackforge/os-ansible-deployment,stackforge/os-ansible-deployment