Document required repository hosts config info
Document required repository hosts config info

Installs must have either the repo-infra_hosts list, or the
openstack_repo_url defined. Otherwise, greenfield installations
following our installation docs will fail.

Change-Id: I116040302e846530895836dd8aab9d4136b110af
Closes-Bug: #1475000

mrda/openstack-ansible,openstack/openstack-ansible,open-switch/infra_openstack-ansible,sti-jans/openstack-ansible,Logan2211/openstack-ansible,manuelfelipe/openstack-ansible,weezer/openstack-ansible,mancdaz/openstack-ansible,shane-c/openstack-ansible,bunchc/openstack-ansible,robb-romans/openstack-ansible,git-harry/os-ansible-deployment,manuelfelipe/openstack-ansible,bunchc/openstack-ansible,yanyao/openstack-ansible,jpmontez/os-ansible-deployment,mancdaz/openstack-ansible,open-switch/infra_openstack-ansible,cfarquhar/openstack-ansible,yanyao/openstack-deployment-liberty,cfarquhar/openstack-ansible,antonym/openstack-ansible,AlphaStaxLLC/os-ansible-deployment,yanyao/newton,antonym/openstack-ansible,stackforge/os-ansible-deployment,jpmontez/os-ansible-deployment,manuelfelipe/openstack-ansible,antonym/openstack-ansible,jpmontez/openstack-ansible,major/os-ansible-deployment,pombredanne/os-ansible-deployment,BjoernT/os-ansible-deployment,achekunov/openstack-ansible,robb-romans/openstack-ansible,major/openstack-ansible,yanyao/openstack-ansible,BjoernT/openstack-ansible,shane-c/openstack-ansible,Logan2211/openstack-ansible,open-switch/infra_openstack-ansible,sti-jans/openstack-ansible,os-cloud/os-ansible-deployment,cloudnull/os-ansible-deployment,mrda/openstack-ansible,mrda/openstack-ansible,yanyao/openstack-deployment-liberty,BjoernT/os-ansible-deployment,AlphaStaxLLC/os-ansible-deployment,major/openstack-ansible,shane-c/openstack-ansible,AlphaStaxLLC/os-ansible-deployment,openstack/openstack-ansible,major/os-ansible-deployment,jpmontez/os-ansible-deployment,cloudnull/os-ansible-deployment,git-harry/os-ansible-deployment,jpmontez/openstack-ansible,pombredanne/os-ansible-deployment,yanyao/newton,BjoernT/os-ansible-deployment,achekunov/openstack-ansible,BjoernT/openstack-ansible,sti-jans/openstack-ansible,pombredanne/os-ansible-deployment,jpmontez/openstack-ansible,yanyao/openstack-deployment-liberty,git-harry/os-ansible-deployment,os-cloud/os-ansible-deployment,stackforge/os-ansible-deployment,achekunov/openstack-ansible,weezer/openstack-ansible