Select default "openstack" plan when no plan-environment.yaml found
Select default "openstack" plan when no plan-environment.yaml found

To avoid having a duplicate plan-environment and capabilities-map in
the root of the container as initially proposed in
https://review.openstack.org/#/c/574753/, we check for a user provided
plan-environment.yaml and when none is found we select the default
"openstack" plan.

Change-Id: Ie26c2df437759a1b99c2498c9ba3039e2ac1c4ae

openstack/tripleo-common,openstack/tripleo-common