missing from docs   Build images fails without : export  DIB_YUM_REPO_CONF="/etc/yum.repos.d/delorean.repo /etc/yum.repos.d/delorean-deps.repo"

Environment:
-------------
instack-0.0.8-dev4.el7.centos.noarch
instack-undercloud-2.1.3-dev219.el7.centos.noarch


Steps: 
------
(1) follow the guide to install liberty rdo-manager http://docs.openstack.org/developer/tripleo-docs/environments/virtual.html 

(2) attempt to build images 

Results: 
---------
build images fails

Workaround run (before build images) : 
----------------------------------------
export  DIB_YUM_REPO_CONF="/etc/yum.repos.d/delorean.repo /etc/yum.repos.d/delorean-deps.repo" fails. 


Expected : command should be added to documentation . 


Results