Add some comments about nova variable definition
Add some comments about nova variable definition

Add comments in playbooks/roles/os_nova/defaults/main.yml,
mainly about nova_virt_type and nova_virt_autodetect.

Change-Id: I51ddfb44adc582c3c0ea47c7a4162881dfa2f7c3

weezer/openstack-ansible,robb-romans/openstack-ansible,mancdaz/openstack-ansible,Logan2211/openstack-ansible,mancdaz/openstack-ansible,mrda/openstack-ansible,openstack/openstack-ansible,yanyao/newton,git-harry/os-ansible-deployment,yanyao/openstack-deployment-liberty,cfarquhar/openstack-ansible,openstack/openstack-ansible,mrda/openstack-ansible,yanyao/newton,stackforge/os-ansible-deployment,yanyao/openstack-ansible,major/openstack-ansible,cfarquhar/openstack-ansible,Logan2211/openstack-ansible,git-harry/os-ansible-deployment,jpmontez/os-ansible-deployment,cloudnull/os-ansible-deployment,yanyao/openstack-deployment-liberty,jpmontez/os-ansible-deployment,open-switch/infra_openstack-ansible,jpmontez/openstack-ansible,BjoernT/os-ansible-deployment,jpmontez/openstack-ansible,jpmontez/os-ansible-deployment,os-cloud/os-ansible-deployment,mrda/openstack-ansible,yanyao/openstack-deployment-liberty,open-switch/infra_openstack-ansible,weezer/openstack-ansible,bunchc/openstack-ansible,BjoernT/openstack-ansible,os-cloud/os-ansible-deployment,major/os-ansible-deployment,yanyao/openstack-ansible,BjoernT/openstack-ansible,major/os-ansible-deployment,jpmontez/openstack-ansible,robb-romans/openstack-ansible,stackforge/os-ansible-deployment,open-switch/infra_openstack-ansible,major/openstack-ansible,bunchc/openstack-ansible,BjoernT/os-ansible-deployment,git-harry/os-ansible-deployment,cloudnull/os-ansible-deployment,BjoernT/os-ansible-deployment